Emit a warning in twine check when Requires-Python is unset

[webknjaz]

Your Environment

N/A

The Issue

While filing an issue about a missing python_requires in yet another project (joeyespo/pytest-watch#125) I suddenly realized that I've filed a few related feature requests in Twine already but when I've checked them, I saw that I never asked to validate its presence (I only asked to compare it with the wheel tags @ #739). So I thought I'd record another possibly useful check to be added.

Steps to Reproduce

N/A

[bhrutledge]

@webknjaz Thanks for the suggestion. Once again, I think this is similar to #430, which is blocked on pypa/packaging#147. However, I have an initial proposal for an alternative approach in #848; I'd appreciate your feedback.

[webknjaz]

It is somewhat similar to the blocked issue but the difference is that this kind of check is not something that the packaging ecosystem is enforcing currently — people can upload dists without Requires-Python, it's not illegal. And the proposed improvement would need to be added on the twine side, not to packaging or wherever the shared code would end up.