chore(proto-plus): update Python version and clean up enums.py

[chalmerlowe]

This PR updates proto-plus 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 3.9.
• Code: Refactored repeated.py to use the walrus operator (PEP 572).
  Documentation: Updated a traceback example in docs/fields.rst to reference Python 3.10.
• Cleanup: Replaced a stale comment regarding Python 3.7 features in proto/enums.py with a tracked TODO referencing Issue Refactor ProtoEnumMeta to use _ignore_ for _pb_options #16911.

Note

Also includes a conditional in the run_single_test.sh file to manage the chicken OR the egg problem of managing failing tests when a unit-3.9 nox session is not present. Because we are migrating handwritten libs one at a time, we are in a transition period were we must still test for 3.9 in libraries that still support it but cannot have a failing presubmit in libraries that don't support it.

Blocking

• This PR is blocked by PR chore(versions): update Python version support to 3.10+ in gapic-generator #16910

[gemini-code-assist]

Code Review

This pull request replaces a descriptive comment in packages/proto-plus/proto/enums.py with a TODO referencing Issue #16911 regarding the use of the ignore attribute. I have no feedback to provide as there are no review comments.

[parthea]

Switching to draft until presubmits are green