MAINT: Update numpy dependency requirement for Python <3.7

[oesteban]

The recently released numpy 1.20 drops Python 3.6

[effigies]

I was just looking into this for nibabel. It's weird that 1.20.0rc1 is getting installed without --pre flags, and also python_requires is not being respected by pip. This seems like it might be resolved by blacklisting pip versions.

[effigies]

Ah, no, apparently it's setuptools not being as smart as pip:

• easy_install (setup.py install) doesn't ignore pre-releases pypa/setuptools#855
• setuptools could respect python_requires pypa/setuptools#1633

Looks like python setup.py install is just no longer recommended.

[oesteban]

Looks like python setup.py install is just no longer recommended.

Yup, that's what I understood from https://snarky.ca/what-the-heck-is-pyproject-toml/

[effigies]

So should we work to support the setup.py install use case for downstream tools? (That's what I'm interpreting this PR as doing.)

[oesteban]

I think so, for a smooth transition to PEP517/8. Also to allow some more time for PEP517 builders to settle in.

[effigies]

But the fix is just to use pip install and let it do dependency resolution.