fix(api-core): drop support for Python 3.9

[chalmerlowe]

This PR updates google-api-core to establish Python 3.10 as the minimum supported version, dropping support for Python 3.9 and below.

Changes

Configuration: Updated pyproject.toml and noxfile.py to require Python 3.10+ and remove references to Python 3.7, 3.8, and 3.9. Updated README.rst and CONTRIBUTING.rst to reflect the new support status.
Cleanup: Removed aiter and anext polyfills in test_bidi_async.py and removed Python < 3.10 polyfills in _python_version_support.py.
Constraints & Dependencies:

• Created constraints-3.10.txt using the old 3.9 constraints as a baseline.
• Bumped grpcio to 1.80.0 and requests to 2.33.0 to avoid issues with pkg_resources and six warnings on Python 3.10.
• Bumped google-auth to 2.35.0 to satisfy async_rest requirements.
  Fixes internal issue: http://b/482126936 🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9, updating documentation, metadata, and CI configurations to set the minimum requirement to Python 3.10. The changes include removing compatibility shims for older Python versions and refactoring logic to utilize features like match statements. Feedback indicates a potential regression in noxfile.py where the new match statement for legacy_proto does not explicitly handle False values, which could lead to assertion failures during testing.

[chalmerlowe]

@parthea

UPDATE: I did not see the approval when I wrote this. Thanks

I believe I have addressed all your comments. Please review. Thanks!