[1] Add tox generation script, but don't use it yet (#3971)

Add:
* tox generation script
* tox template
* script for generating tox and CI yamls in one go
* readme for the script

In this PR, the script is set to ignore all integrations, so no tox
configuration is actually added. However, it's still the script actually
generating the real `tox.ini` from the `tox.jinja` template.

See follow-up PRs for more.

---------

Co-authored-by: Daniel Szoke <[email protected]>

scripts/generate-test-files.sh

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+#!/bin/sh
+
+# This script generates tox.ini and CI YAML files in one go.
+
+set -xe
+
+cd "$(dirname "$0")"
+
+python -m venv toxgen.venv
+. toxgen.venv/bin/activate
+
+pip install -e ..
+pip install -r populate_tox/requirements.txt
+pip install -r split_tox_gh_actions/requirements.txt
+
+python populate_tox/populate_tox.py
+python split_tox_gh_actions/split_tox_gh_actions.py

scripts/populate_tox/README.md

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+# Populate Tox
+
+We integrate with a number of frameworks and libraries and have a test suite for
+each. The tests run against different versions of the framework/library to make
+sure we support everything we claim to.
+
+This `populate_tox.py` script is responsible for picking reasonable versions to
+test automatically and generating parts of `tox.ini` to capture this.
+
+## How it works
+
+There is a template in this directory called `tox.jinja` which contains a
+combination of hardcoded and generated entries.
+
+The `populate_tox.py` script fills out the auto-generated part of that template.
+It does this by querying PyPI for each framework's package and its metadata and
+then determining which versions make sense to test to get good coverage.
+
+The lowest supported and latest version of a framework are always tested, with
+a number of releases in between:
+- If the package has majors, we pick the highest version of each major. For the
+  latest major, we also pick the lowest version in that major.
+- If the package doesn't have multiple majors, we pick two versions in between
+  lowest and highest.
+
+#### Caveats
+
+- Make sure the integration name is the same everywhere. If it consists of
+  multiple words, use an underscore instead of a hyphen.
+
+## Defining constraints
+
+The `TEST_SUITE_CONFIG` dictionary defines, for each integration test suite,
+the main package (framework, library) to test with; any additional test
+dependencies, optionally gated behind specific conditions; and optionally
+the Python versions to test on.
+
+Constraints are defined using the format specified below. The following sections describe each key.
+
+```
+integration_name: {
+    "package": name_of_main_package_on_pypi,
+    "deps": {
+         rule1: [package1, package2, ...],
+         rule2: [package3, package4, ...],
+     },
+     "python": python_version_specifier,
+}
+```
+
+### `package`
+
+The name of the third party package as it's listed on PyPI. The script will
+be picking different versions of this package to test.
+
+This key is mandatory.
+
+### `deps`
+
+The test dependencies of the test suite. They're defined as a dictionary of
+`rule: [package1, package2, ...]` key-value pairs. All packages
+in the package list of a rule will be installed as long as the rule applies.
+
+`rule`s are predefined. Each `rule` must be one of the following:
+  - `*`: packages will be always installed
+  - a version specifier on the main package (e.g. `<=0.32`): packages will only
+    be installed if the main package falls into the version bounds specified
+  - specific Python version(s) in the form `py3.8,py3.9`: packages will only be
+    installed if the Python version matches one from the list
+
+Rules can be used to specify version bounds on older versions of the main
+package's dependencies, for example. If e.g. Flask tests generally need
+Werkzeug and don't care about its version, but Flask older than 3.0 needs
+a specific Werkzeug version to work, you can say:
+
+```python
+"flask": {
+    "deps": {
+        "*": ["Werkzeug"],
+        "<3.0": ["Werkzeug<2.1.0"],
+    },
+    ...
+}
+```
+
+If you need to install a specific version of a secondary dependency on specific
+Python versions, you can say:
+
+```python
+"celery": {
+    "deps": {
+        "*": ["newrelic", "redis"],
+        "py3.7": ["importlib-metadata<5.0"],
+    },
+    ...
+}
+```
+This key is optional.
+
+### `python`
+
+Sometimes, the whole test suite should only run on specific Python versions.
+This can be achieved via the `python` key, which expects a version specifier.
+
+For example, if you want AIOHTTP tests to only run on Python 3.7+, you can say:
+
+```python
+"aiohttp": {
+    "python": ">=3.7",
+    ...
+}
+```
+
+The `python` key is optional, and when possible, it should be omitted. The script
+should automatically detect which Python versions the package supports.
+However, if a package has broken
+metadata or the SDK is explicitly not supporting some packages on specific
+Python versions (because of, for example, broken context vars), the `python`
+key can be used.
+
+
+## How-Tos
+
+### Add a new test suite
+
+1. Add the minimum supported version of the framework/library to `_MIN_VERSIONS`
+   in `integrations/__init__.py`. This should be the lowest version of the
+   framework that we can guarantee works with the SDK. If you've just added the
+   integration, you should generally set this to the latest version of the framework
+   at the time.
+2. Add the integration and any constraints to `TEST_SUITE_CONFIG`. See the
+   "Defining constraints" section for the format.
+3. Add the integration to one of the groups in the `GROUPS` dictionary in
+   `scripts/split_tox_gh_actions/split_tox_gh_actions.py`.
+4. Add the `TESTPATH` for the test suite in `tox.jinja`'s `setenv` section.
+5. Run `scripts/generate-test-files.sh` and commit the changes.
+
+### Migrate a test suite to populate_tox.py
+
+A handful of integration test suites are still hardcoded. The goal is to migrate
+them all to `populate_tox.py` over time.
+
+1. Remove the integration from the `IGNORE` list in `populate_tox.py`.
+2. Remove the hardcoded entries for the integration from the `envlist` and `deps` sections of `tox.jinja`.
+3. Run `scripts/generate-test-files.sh`.
+4. Run the test suite, either locally or by creating a PR.
+5. Address any test failures that happen.
+
+You might have to introduce additional version bounds on the dependencies of the
+package. Try to determine the source of the failure and address it.
+
+Common scenarios:
+- An old version of the tested package installs a dependency without defining
+  an upper version bound on it. A new version of the dependency is installed that
+  is incompatible with the package. In this case you need to determine which
+  versions of the dependency don't contain the breaking change and restrict this
+  in `TEST_SUITE_CONFIG`.
+- Tests are failing on an old Python version. In this case first double-check
+  whether we were even testing them on that version in the original `tox.ini`.

scripts/populate_tox/config.py

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+# The TEST_SUITE_CONFIG dictionary defines, for each integration test suite,
+# the main package (framework, library) to test with; any additional test
+# dependencies, optionally gated behind specific conditions; and optionally
+# the Python versions to test on.
+#
+# See scripts/populate_tox/README.md for more info on the format and examples.
+
+TEST_SUITE_CONFIG = {}