Simplify my config for mypy

[alexwlchan]

I have a whole bunch of mypy config which I'm copying from repo to repo, which I first copied from urllib3.

This search will find all instances of it: https://github.com/search?type=code&q=%28org%3AFlickr-Foundation%20OR%20org%3Aalexwlchan%29%20NOT%20is%3Aarchived%20no_implicit_optional

In practice I just want mypy to run in strict mode, and not worry too much about the specifics. Looking at the documentation, I think it would be simpler to replace all those lines with strict = true and I'll get the same effect, plus get opted in to future checks if/when mypy adds them.