allow Python, dependency versions checks without upgrading api_core

The preferred method is still to sue the code in api_core, but we have
a (repetitive) fallback in case users haven't installed the new version that
implements that functionality.

Author: vchudnov-g

gapic/templates/%namespace/%name_%version/%sub/__init__.py.j2

@@ -10,14 +10,61 @@ __version__ = package_version.__version__
 
 import google.api_core as api_core
 
-{# How do we get the name of the PyPI path into this template? We
-   want the string arguments below to be something like
-   "google-cloud-foo (google.cloud.foo)", where the name outside the
-   parentheses is the PyPI package name, and the the name inside the
-   parentheses is the qualified Python package name installed. #}
-api_core.check_python_version("{{package_path}}")
-api_core.check_dependency_versions("{{package_path}}")
+try:
+  api_core.check_python_version("{{package_path}}")
+  api_core.check_dependency_versions("{{package_path}}")
+except AttributeError:
+{# TODO: Remove this try-catch when we require api-core at a version that
+   supports the changes in https://github.com/googleapis/python-api-core/pull/832
 
+    In the meantime, please ensure the functionality here mirrors the
+    equivalent functionality in api_core, in those two functions above.  
+-#}
+  # An older version of api_core is installed, which does not define the
+  # functions above. We do equivalent checks manually.
+
+  import logging
+
+  _py_version_str = sys.version.split()[0]
+  _package_label = "{{package_path}}"
+  if sys.version_info < (3, 9):
+    logging.warning("You are using a non-supported Python version " +
+            f"({_py_version_str}).  Google will not post any further " +
+            f"updates to {_package_label} supporting this Python version. " +
+            "Please upgrade to the latest Python version, or at " +
+            f"least to Python 3.9, and then update {_package_label}.")
+  if sys.version_info[:2] == (3, 9):
+    logging.warning(f"You are using a Python version ({_py_version_str}) " +
+            f"which Google will stop supporting in {_package_label} when " +
+            "it reaches its end of life (October 2025). Please " +
+            "upgrade to the latest Python version, or at " +
+            "least Python 3.10, before then, and " +
+            f"then update {_package_label}. "
+
+  import pkg_resources
+  from packaging.version import parse as parse_version
+
+  def _get_version(dependency_name):
+    version_string = pkg_resources.get_distribution(dependency_name).version
+    return parse_version(version_string)
+
+  _dependency_package = "google.protobuf"
+  _version_used = _get_version(_dependency_package)
+  _next_supported_version = "4.25.8"
+  if _version_used < parse_version(_next_supported_version):
+    logging.warning(f"DEPRECATION: Package {_package_label} depends on " +
+            f"{_dependency_package}, currently installed at version " +
+            f"{_version_used.__str__}. Future updates to " +
+            f"{_package_label} will require {_dependency_package} at " +
+            f"version {_next_supported_version} or higher. Please ensure " +
+            "that either (a) your Python environment doesn't pin the " +
+            f"version of {_dependency_package}, so that updates to " +
+            f"{_package_label} can require the higher version, or " +
+            "(b) you manually update your Python environment to use at " +
+            f"least version {_next_supported_version} of " +
+            f"{_dependency_package}."
+      
+  
 {#  Import subpackages. -#}
 {% for subpackage, _ in api.subpackages|dictsort %}
 from . import {{ subpackage }}