Add PEP 621 project table parsing and validation

[chrisjsewell]

Heya,

This module takes a pyproject.toml dict (pre-parsed by a TOML library) and parses/validates the [project] table, according to PEP 621.

I feel this could be generically useful, for packaging tools?

If you agree, I can clean up the code a bit and add more tests, etc

[brettcannon]

@chrisjsewell can you please open an issue for us to discuss your proposed feature request?

[chrisjsewell]

done (#647) 👍