Enable warn_unreachable = True for mypyc and all other files as well

[wyattscarpenter]

Achieves a todo, enabling warn_unreachable = True for mypyc and all other files as well

[wyattscarpenter]

I am now waiting on mypyc/mypyc#1101 to be resolved to finish this issue, after which I will have to rebase this branch onto master to get the resolution.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sterliakov]

Wow, only one hit? That should be easy to resolve...

[wyattscarpenter]

@stanislavlevin, yep! Only one hit for this project 🥳!Unfortunately (so to speak), deciding what to do about that is best left to people more familiar with mypyc than me.

I could simply comment out this code, which I might do eventually, but if someone with a better idea about what it should do comes along, I will defer to them.