Drop support for EOL pythons?

[h-vetinari]

I think we should drop the markers for 3.5 and probably 3.6, and just raise python_requires appropriately. Those on old python versions will still get functioning oldest-supported-numpy packages, and almost by definition, those old language versions will not receive any new architecture support. However, they still have a maintenance cost, as seen in cases like #71, so I think we should drop them.

Python 3.7 is also EOL this month, but it's a borderline case.

[h-vetinari]

Python 3.5 has also already been dropped from test_dependencies.py...

[rgommers]

That sounds okay to me indeed. We should keep 3.7 for a little while longer.