pyproject.toml support

[ajslater]

Fixes: #220

This enables pyproject.toml support for radon.

Decisions I made the maintainer should be aware of before merging:

• Automatic pyproject.toml support for python 3.11+
• Optional pyproject.toml support for python <3.11 if you install with:

  pip install radon[toml]

• Silently ignores pyproject.toml if:
  • no pyproject.toml file is found
  • no toml parsing library is available
• Raises an exception if:
  • toml file is found and is parsed incorrectly.

I'm happy to change anything that isn't quite in the spirit of radon.

[VildMedPap]

I just want to say I +1 this feature.

[rubik]

@ajslater Thanks for the contribution. It's quite useful and I finally got around to reviewing it.