typing: fix the typehint for `skip` arg on `osutil.listdir()` to be optional The cffi and pure implementations both default to `None`, and the args are parsed in C with `"y#|OO:listdir"`, with the C variable initialized to NULL.

typing: add type hints to mpatch implementations Again, using `merge-pyi` to apply the stubs in cext and then manually type the private methods. The generated stub without these hints inferred very little, and the stuff it did was wrong.