✨ Update Python parser and support dynamic version and description

[dd]

Updated the Python parser:

• now supports dynamic fields (version and description);
• for dynamic version in Python projects, added the --version-command option to specify a command that returns the project version;
• if description is dynamic - it is now taken from the first line of the README;
• added descriptions for new mechanics to docs;

I added smol-toml because it supports the TOML 1.0.0 spec, unlike the toml package which only supports up to 0.4.0. That said, I kept toml in place for other parsers — though maybe it’s worth replacing it everywhere eventually.

Also added tests for all of that.

The parser is now more in line with PEP 621 and modern pyproject.toml setups.