fix(google-auth-oauthlib): Drop support for Python 3.9

[chalmerlowe]

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

Changes

• Configuration: Updated setup.py and noxfile.py to require Python 3.10+ and remove references to Python 3.9.
• Documentation: Updated README.rst and CONTRIBUTING.rst to reflect the new supported and unsupported Python versions.
• Constraints: Updated constraints-3.10.txt with minimum versions from constraints-3.9.txt and deleted constraints-3.9.txt.

Fixes internal issue: http://b/482126936 🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9 across the repository, updating the documentation, noxfile.py, and setup.py to set Python 3.10 as the new minimum supported version. Feedback was provided regarding the removal of constraints-3.9.txt, suggesting that a constraints-3.10.txt file should be added to maintain lower-bound dependency testing for the updated minimum version.

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packages/google-auth-oauthlib/testing/constraints-3.9.txt (1-10)

The constraints-3.9.txt file was removed as part of dropping support for Python 3.9. However, a corresponding constraints-3.10.txt file should be added to ensure that lower-bound dependency testing continues for the new minimum supported Python version (3.10), as indicated by the comments in the removed file. If the project's policy is to test the lower bounds of dependencies for the minimum supported Python version, this file is necessary for the CI/CD pipeline.

[chalmerlowe]

Code Review

This pull request drops support for Python 3.9 across the repository, updating the documentation, noxfile.py, and setup.py to set Python 3.10 as the new minimum supported version. Feedback was provided regarding the removal of constraints-3.9.txt, suggesting that a constraints-3.10.txt file should be added to maintain lower-bound dependency testing for the updated minimum version.

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Resolved. There is a 3.10 file.