Add --warn-unused-strictness-exceptions flag

[ilinum]

Warn about strictness flags unnecessarily disabled for particular modules.
If a flag is enabled globally (either explicitly or by default) and it is
disabled for a particular module that does not have errors related to this
flag mypy will output an error.

Currently, this is implemented only for --disallow-any-generics but it should be
very straightforward to support other strictness flags.

Fixes#4018

[ilinum]

The tests that this PR is failing were introduced in #3952.

It's definitely a bug in collect_any_types that was there before this PR: if disallow_any=generics was enabled for a file that had a recursive definition (e.g. a recursive NamedTuple), it would crash.

@ilevkivskyi I looked at #3952 but I'm having trouble figuring it out on how to fix this particular problem. Can you give me some pointers?

[ilevkivskyi]

collect_any_types happens too soon. Recursive types are "fixed" in patches after third pass. There are two options:

• Quick and simple fix: just move to a later stage, either append patches (be sure to place them at the very end) or move to checker if possible.
• A bit more long-term solution (that will still work if recursive types are supported) but harder to implement is to track visited type nodes while collecting.

I would recommend the first option, unless @JukkaL has a better idea.

[gvanrossum]

Hey @ilinum shall we abandon this? Or do you have time to bring it back into shape?

[ilinum]

No, I don't think I have enough time to work on this. If someone wants to take this over, go ahead!