Drop Python 3.6 support (from v0.15+)

[CasperWA]

It seems about time Python 3.6 support is dropped.

Some points:

• We can't use deferred type hints with Python 3.6.
• numpy and pymatgen have dropped Python 3.6 support.

[ml-evs]

This should also simplify much of our dependency handling for e.g. pymatgen and numpy

[ml-evs]

Any objection to dropping it before v0.14.0?

[CasperWA]

Any objection to dropping it before v0.14.0?

Not really. But I think we should list all considerations in the OP first, before dropping support.
It might be nice with a grace period as well (although I understand your POV considering #748).

[ml-evs]

I have made #750 to actually do this when we are ready.

I honestly don't see any downsides at the moment. I don't think we want to support another whole minor version cycle (v0.14.x) with Python 3.6 so now seems like a sensible time to drop it. Also before the project gets advertised more widely...

[CasperWA]

Any comments on this @markus1978 or @shyamd?

[shyamd]

We've dropped it in MP stuff. Numpy no longer supports 3.6, so it's hard to keep supporting it.

[CasperWA]

So on Slack we've discussed a bit and agreed to have a small grace period, since the immediate issue has been resolved and it is not crucial anymore to drop Python 3.6 support. While this package is maybe not widely used (especially in production) yet, the grace period might be a bit grand - but it's good practice, and it seems better to give notice before dropping it. Even if it's just a single user that is hit.