Tag Linux wheels as manylinux so PyPI accepts them in publish

[Copilot]

The publish job failed because twine upload got 400 Bad Request from PyPI on pythonmonkey-1.4.0-cp310-cp310-linux_aarch64.whl. PyPI rejects wheels with a plain linux_* platform tag. Only the cp38 wheels were tagged manylinux_2_31_*, so every other Linux wheel (cp39–cp315) carried the rejected tag. I did not find out why cp38 differs.

• Workflow change (.github/workflows/test-and-publish.yaml)
  • Added a "Tag the Linux wheels as manylinux" step to build-and-test. It runs on Ubuntu runners, before the wheels are uploaded as artifacts.
  • The step renames *-linux_*.whl to *-manylinux_2_31_*.whl. The wheels are built in the ubuntu:20.04 container (glibc 2.31), which matches the tag the cp38 wheels already get. It mirrors the existing macOS retagging step.

pythonmonkey-1.4.0-cp310-cp310-linux_x86_64.whl
  -> pythonmonkey-1.4.0-cp310-cp310-manylinux_2_31_x86_64.whl

For review

• This is a filename-only retag and I have not verified it with auditwheel. Please confirm the wheels are compatible with manylinux_2_31.
• The retag happens before the pytest step, which runs pip install ./dist/*, so that step now installs the renamed wheel. pip's tag check should accept it, since a glibc 2.31 container satisfies manylinux_2_31.