Upgrade codebase to recent changes in Python ecosystem

[benoit74]

Fix #23

Changes:

• Drop support for Python 3.7 (EOL)
• Add support for Python 3.11 and 3.12
• Migrate from setup.py to pyproject.toml (PEP 518), especially since pip is going to drop support for setup.py
• Replace deprecated (in python 3.12) cgi package with multipart external dependency
• Remove constraint idna<3 now that requests package is working ok with idna 3
• Add contributing instructions (running tests and building sdist/wheel)
• Remove unused import pkg_resources (which is removed in Python 3.12)
• Upgrade Github actions to latest versions
• Upgrade CI to run style checks and upload code coverage only for one Python version (and not once per Python version)
  • lowest version chosen (3.8) without strong arguments, could have chosen highest version (3.12) as well

[benoit74]

@ikreymer this is ready for review, I can't assign a reviewer myself.

I hope I did not break anything ^^

[benoit74]

Would be glad if someone could have a look at this