Remove -> None annotation from __init__

[jlowin]

Last month, mypy adopted a change that makes the -> None return annotation for otherwise-annotated __init__() signatures unnecessary.

This PR implements that change in our codebase, since I think it's much cleaner (__init__ is ALWAYS -> None).

However, this is a stylistic question so I put it up for debate.

Here's the documentation:
https://mypy.readthedocs.io/en/latest/class_basics.html#annotating-init-methods

And here's the 4 years of discussion:
python/mypy#604

[cicdw]

Question that I just can't ignore: given that no other PRs were being merged, why were 3 force pushes necessary for this?

[cicdw]

I'm cool with this

[jlowin]

It was my obsession with making this a single commit. A couple of weird mypy breakages that had to be fixed multiple times, so each time I squashed and force-pushed.