REL: add PEP 621 (project metadata in pyproject.toml) support

[rgommers]

Static metadata in a format that all packaging tools understand will be beneficial in the long term. PEP 621 (https://www.python.org/dev/peps/pep-0621/) became final last month, so let's support it.

pip doesn't have support yet it looks like, so can't really test. But this is all straightforward, should be correct (and can't break anything if CI passes). I based this on both the PEP itself and on flit's implementation.

@takluyver would you mind having a quick look at this?

[takluyver]

Looks OK at a quick glance. I think the idea is that build backends (like setuptools) will add support, and then you can specify metadata like this instead of in setup.py or setup.cfg. Until then, there may be little concrete benefit to adding this (and there's always some risk that some detail won't quite fit with what setuptools expects when it does add support).

[rgommers]

Thanks Thomas.

Until then, there may be little concrete benefit to adding this (and there's always some risk that some detail won't quite fit with what setuptools expects when it does add support).

Makes sense in principle. However, I'm planning to move away from distutils / setuptools, and this is a small step in that direction. And we're anyway putting an upper bound on setuptools so if this PR ends up in SciPy 1.7.0 and then setuptools decided to do something odd, nothing will break. pip is another story of course, but that won't affect releases.

[larsoner]

LGTM, feel free to merge if you're done @rgommers

[rgommers]

Thanks Eric and other reviewers, done!