feat: support pyproject.toml in compile_pip_requirements

[aignas]

With this PR we can also use pyproject.toml in addition
to requirements.in which helps in making the requirements
in a more structured form. For example, we could parse the
toml itself and create aliases in the hub repos only for the
packages outlined in the pyproject.toml file. The same for
gazelle, we could restrict gazelle_python.yaml contents
to only the dependencies listed in pyproject.toml.

Examples can be migrated once we agree on the interface.

Summary:

• feat: support pyproject.toml in compile_pip_requirements
• chore: use pyproject.toml for sphinx doc requirements

[groodt]

LGTM

I think in the description, could be worth clarifying that it's support PEP-621 project.dependencies https://packaging.python.org/en/latest/specifications/declaring-project-metadata/#declaring-project-metadata only. It shouldn't imply support of any other TOML tables that may exist, or tool specific tables such as tool.black or tool.poetry etc

[rickeylev]

LGTM. I pushed a small update to the src arg doc.

The only comment, more question, I have is: should src default to requirements.in or pyproject.toml? If I understand correctly, toml files are the newer, better way?

[aignas]

I have changed the code to default to pyproject.toml as I agree that people who are new to rules_python should use by default new standards, but that may cause issues for older users who were relying on default behaviour.

If you are OK with the changes, explanation, feel free to merge it during your working hours.