stubtest: add tests (#8380)

* stubtest: move into mypy

* stubtest: add entry point to setup.py

* stubtest: use mypy.utils.check_python_version

* stubtest: split up main to make it easier to test

* stubtest: improvements to signature checking

Fixes some false negatives and a minor false positive. Makes the logic
more readable and improve comments.

* stubtest: [minor] follow project style / track coverage better

* stubtest: [minor] output filename more consistently

* stubtest: [minor] remove no longer necessary optional

* stubtest: fix module level variables missing at runtime

Dumb mistake causing false negatives, mainly seems to surface a lot of
platform differences

* stubtest: handle compile errors

* stubtest: [minor] remove black's commas

* stubtest: [minor] handle a case in get_mypy_type_of_runtime_value

Doesn't make a difference to typeshed

* stubtest: add tests

* stubtest: [minor] remove black's colon spaces

To comply with project style

* stubtest: [minor] catch more warnings

* stubtest: replace use of find for Windows compatibility

This is nicer too

* teststubtest: NamedTemporaryFile doesn't work on Windows

* stubtest: add annotation to help mypyc out

* stubtest: [minor] make str(signature) deterministic

* mypyc: exclude stubtest.py

* stubtest: fix LiteralType misuse for mypyc

EAFP, since bytes and enums should work, and default value error
messages can be more informative with literal types

* stubtest: work around a bug in early versions of py35

Author: hauntsaninja