Fix type hints for unorderable_list_difference()
It claimed that the arguments were `Sequence` types (immutable), but the function removes elements from them, so they must be `MutableSequence` types. Specifically, it calls `pop()` on the first argument and `remove()` on the second argument.
See the function implementation for details:
https://github.com/python/cpython/blame/0b06d2482d77e02c5d40e221f6046c9c355458b2/Lib/unittest/util.py#L98-L113
Rob Percival