Ditch python2

[league]

This was a branch, but I just updated it from master and started a PR to more easily see and discuss changes.

(I was about to delete some __future__ stuff when I remembered to look for this branch.)

[league]

I think all of the clean-ups in #178 are here now, except the ones that are more complicated:

• removing block.py
• ring vs ring2

But commits in this PR (or that previously made it to master) include these clean-ups:

• remove dtype.py
• pypy stuff
• commented-out code in ring.py
• remove test_fft.cu

…in addition to the Python2 cleanups.

[jaycedowell]

I recall problems when I tried to remove block.py but I don't remember exactly what they were. It's a shame that those logs from #178 have expired and been removed.

[jaycedowell]

When I was working on 8cff5c2 I noticed that there was another Python2-ism buried in test_scripts.py. There might be other things labeled as "Python2 catch" in the code, specifically related to bytes -> str -> bytes conversion that were missed.

[jaycedowell]

Should we put a "needs Python 3.6+" message somewhere? That's the only thing I can thing of that this might be missing.

Added in b5f503a.

[jaycedowell]

Should we also change configure to check if Python is >=3.6?

Add in e01355f.

[jaycedowell]

I fixed most of the things that I noticed but there are a couple left.

[jaycedowell]

That's everything now.