This makes testing/ actually pick up the pytest imports -- otherwise
they are opaque and we don't actually test the types.

A single run is also a bit faster and simpler. The original reason why
we split it is no longer relevant (we fixed the problems).

Release notes: http://mypy-lang.blogspot.com/2019/07/mypy-0720-released.html

isinstance() accepts it and some code does pass a tuple.

Fixup for commit 55a570e.

It's __repr__ explicitly handles it so allow it.

Not needed since 4d49ba6.

Since 4622c28, _finalizers cannot
contain Nones or tuples, so these checks are not longer needed.

It seems to have been added in pytest-dev#1439 to fix pytest-dev#1178.

This was only relevant for Python 2 where it was tempting to use str (==
bytes) literals instead of unicode literals. In Python 3, it is unlikely
that anyone passes bytes to these functions.