Remove support for Python 3.6

[fangerer]

As of December 23rd, 2021, Python 3.6 has reached end of life (https://devguide.python.org/devcycle/#end-of-life-branches).
We should cleanup our code and officially remove support for 3.6 since that simplifies implementation of several features.

[antocuni]

#338 removes support for 3.6, but the only 3.6-specific hack which I could find and remove was this:
fafec57

did you have other cleanups in mind?

[fangerer]

I think that's it for now. My initial reason for dropping 3.6 support was because of PR #251 . I think it's soon a good time to restart work on that.

[mattip]

The metadata should reflect what python versions are supported, like NumPy does

[fangerer]

Afaik, this issue has been resolved. So, closing it.