chore: migrate to uv+ruff

[weiiwang01]

Overview

Migrate the Python project to use uv, tox-uv, and ruff for building, testing and linting.

The changes include:

• Merge requirements.txt into pyproject.toml
• Migrate tox.ini to tox.toml
• Replace tox with tox-uv
• Use ruff for linting and formatting, replacing flake8, pylint, black, and isort
• Fix some linting issue discovered by ruff

Rationale

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated
• A change artifact was added for user-relevant changes in docs/release-notes/artifacts.
• The PR is tagged with appropriate label (urgent, trivial, complex)
• The changelog is updated with user-relevant changes in the format of keep a changelog v1.1.0

[Thanhphan1147]

LGTM, thanks!

[weiiwang01]

@copilot I want you to modify this pull request.

The first thing you need to do is add a new entry to docs/changelog.md. The changelog entry should be short and should describe the changes introduced by this pull request.

The second thing you need to do is update CONTRIBUTING.md to explain how to use uv and tox-uv for development. For your reference, here's a section I copied from another project. In this reference, it's using uv sync --all-groups but if you are seeing tool.uv.conflicts defined in pyproject.toml use uv sync instead.

## Developing

To make contributions to this charm, you'll need a working [development setup](https://documentation.ubuntu.com/juju/3.6/howto/manage-your-deployment/).

The code for this charm can be downloaded as follows:

```
git clone https://github.com/canonical/pollen-operator
```

Make sure to install [uv](https://docs.astral.sh/uv/), for example:

```sh
sudo snap install astral-uv --classic
```

Then install `tox` with extensions, as well as a range of Python versions:

```sh
uv tool install tox --with tox-uv
uv tool update-shell
```

To create an environment for development, use:

```shell
uv sync --all-groups
source venv/bin/activate
```

For all your modifications, make minimal changes to complete the task.

[Copilot]

@weiiwang01 I've opened a new pull request, #227, to work on those changes. Once the pull request is ready, I'll request review from you.

[Thanhphan1147]

@weiiwang01 I think we should not touch and code in legacy/ and add ignore rules for the different tools we have. Since that code is brought over from the old charm and we need to keep it unchanged to ensure backwards compatibility. We've also decided that that code will not be maintained so it's fine to exclude it from our lint suite

Please ignore this I misread tests/unit/legacy, which is fine 😄