Move pytest config to pyproject.toml

[matdibu]

Summary

Move pytest config from pytest.ini to pyproject.toml

This was already done for the betterproto2 package, now betterproto2_compiler does the same

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[AdrienVannson]

Thank you! Actually, I saw that the whole file was outdated, so it was better to simply remove it: #166