Remove support for Python 3.7

[tomwhite]

NumPy 1.22 no longer supports Python 3.7, so we should consider removing support for it in the next release of sgkit.

This would be consistent with https://numpy.org/neps/nep-0029-deprecation_policy.html, which says Python 3.7 can be dropped from Dec 26, 2021.

[jeromekelleher]

Seems quite early to drop 3.7 tbh - is there much to be gained? To me there aren't any standout features in 3.8 making it worthwhile, and we're unlikely to (shouldn't?) depend on features required numpy 1.22 for a long time.

I don't feel strongly about it, though.

[tomwhite]

No particular rush, just wanted to open it for discussion. Installing Python 3.8 envs using conda is easy enough on most platforms so I suspect that shouldn't be a blocker.

[tomwhite]

Well that was quick. Our upstream tests just failed because xarray has already dropped Python 3.7.

[tomwhite]

Pandas 1.4.0 does not support Python 3.7.

[tomwhite]

The build is now failing on Python 3.7 (https://github.com/pystatgen/sgkit/actions/runs/3186604485), so I'm thinking it's probably time to remove support for it.

[jeromekelleher]

No point in swimming against the tide I guess.