The stubs for `unittest.TestCase.assertCountEqual()` specify the parameters are Sequences; in fact the method works fine with Iterables, as the first thing the method does is convert them to lists. [1] The docs do say sequences, but it appears there is no reason they cannot be Iterables.
[1]: https://github.com/python/cpython/blob/master/Lib/unittest/case.py#L1156
According to the documentation in the typing module, TypeVars cannot
have only a single constraint. Attempting to do so will actually result
in an exception at runtime. (However, this error is currently ignored
by mypy -- see https://github.com/python/mypy/pull/2626 for a related
pending pull request).
This commit changes all instances of TypeVars using a single constraint
(e.g. `T = TypeVar('T', Foo)`) to use bounds instead (e.g.
`T = TypeVar('T', bound=Foo)`.
This seems to be the correct fix for plistlib after reading the module
docs, but it's less obvious this is correct for unittest. The unittest
module originally had `_FT = TypeVar('_FT', Callable[[Any], Any])` -- an
alternative fix would have been to do `_FT = Callable[[Any], Any]`.
Although I'm not entirely sure what it means to have a bound be a
Callable, I decided to make the assumption that the original authors
probably meant to use TypeVars instead of type aliases for a reason
(possibly to handle classes implementing `__call__`?)
