What was changed

Drop Python 3.8 support and confirm Python 3.13 support

Specifically this included:

• Update CI GH workflow to do 3.9/3.13 instead of 3.8/3.12
• Update binary build and bench GH workflows to do 3.13
• Remove outdated workarounds for older Poetry and older Python versions in CI
• Create a new branch in features repo named python-version-upgrade to pass CI
• Update minimum Python versions in README
• Update code to support protobuf 5.x
  • Basically this was just doing a poetry update for protobuf and some minor tweaks, but it means we now test proto 3.x and proto 5.x in CI, but not 4.x
• Update cibuildwheel and align grpcio and `grpcio-tools
• Add protoc install step to wheel building since it was removed as part of pyproject.toml: Removed protoc-wheel dependency #684 and we didn't notice until now it broke wheel building
• Update fix-wheel.py script to unzip wheel to fix it instead of wheel unpack which was failing on macos for strange hash mismatch
• Updated wheel and bridge to have minimum 3.9 support instead of 3.8
• Exclude os.stat from sandbox restrictions since as of gh-106922: Support multi-line error locations in traceback (attempt 2) python/cpython#112097 it appears that is invoked even when displaying errors
• Update test teardown to, in 3.9, swallow certain loop close errors and disable pytest-asyncio's creation of a post-teardown loop creation since they were breaking in strange ways
• Skip all unfinished-handler tests in 3.9 since they were reliably causing issues (instead of sometimes flaking as in all other Python versions). This needs to be investigated when the unfinished-handler test flakes are investigated in general.

After the PR is approved, I will change the required passing CI steps to be 3.9/3.13.

Checklist

1. Closes [Feature Request] Drop 3.8 support, confirm 3.13 support #672