Add build_requires option to pyproject.toml

[drasmuss]

Motivation and context:

This lets projects specify additional build dependencies if needed.

Since it's now possible that a user would want to template pyproject.toml but not pre-commit-config.yaml, I reworked the validation that used to require both be present if either was present.

Interactions with other PRs:

Used in nengo/keras-lmu#42, where it is needed to add the oldest-supported-numpy dependency.

How has this been tested?

Added a test, see test_generate_bones.py::test_pyproject_toml.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• New feature (non-breaking change which adds functionality)

Checklist:

• I have read the CONTRIBUTING.rst document.
• I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• I have run the test suite locally and all tests passed.

Still to do:

• Update documentation

[drasmuss]

This didn't end up fixing the problem I hoped it would fix in nengo/keras-lmu#42, so there's no particular motivation to merge this in now. But it works, so I'll leave it up in case we want to come back to this in the future.