Drop support for Python 3.5

[srittau]

Python 3.5 EOL was on 2020-09-30.

[hauntsaninja]

Thanks for doing this! Looks good as far as I can tell.

I don't have a good sense of when the right time to start burying 3.5's corpse is, so trusting you on that. It looks like we waited four months past EOL for 3.4.
Note @JukkaL expressed some reluctance recently: python/mypy#9536 (comment) For mypy, 3.5 seems to account for only a small share of downloads (although we don't know what version of Python people are targetting).
In terms of precedent setting, time will tell, but I think we might want to be a little slower when 3.6's time comes.

[JukkaL]

The decision to drop 3.5 support in typeshed can be orthogonal to the decision to drop 3.5 support in mypy, I think. Type checking Python 3.5 code will be possible (though somewhat limited) even without explicit support in typeshed. Mypy just won't be able to flag uses of Python 3.6 library features when targeting 3.5. Almost all changes in 3.6 seem to add new functionality instead of removing existing functionality or modifying things in incompatible ways.

Starting from the next mypy release, I'm fine with 3.5 support being incomplete. The main thing is being able to run mypy using Python 3.5 when that's what ships with the operating system.

[JelleZijlstra]

I'm OK with doing this, but there's a ton of merge conflicts.

[srittau]

@JelleZijlstra I rebased the PR on current master.

[JelleZijlstra]

Looks good, but do you know what's going on with mypy-primer here?

[JelleZijlstra]

Oh, I guess that might just have been because of a bad merge that produced syntax errors in a few files.