Adds support for Path and Pathlike objects to TemplateStream.dump.

My understanding of the existing code is:

1. Strings are treated as file paths
2. Other valid objects implement a write(value: bytes) -> None or writelines(Iterable[bytes]) -> None method.
3. Invalid objects raise an AttributeError, e.g. AttributeError: 'NoneType' object has no attribute 'write'

My change would modify the behavior to:

1. Any object that the builtin open can handle as a file path is a valid file path. This includes the requested pathlib.Path and PathLike objects.
2. This functionality is unchanged.
3. Invalid objects still raise an AttributeError with a slightly more informative message AttributeError: 'NoneType' object has no attribute 'write' or 'writelines'

Possible Breaking Changes (No tests guarantee this functionality):

1. Any object that isn't a string but open is capable of handling.
2. Any equality checks on the error message for the AttributeError

This does not address #1983 the only changes to the typing is the inclusion of the full set of types that open implements.

fixes #2039

Desired Feedback

1. This is my first PR for a pallets project. Any feedback is welcome.
2. The tests are very comprehensive, and may be too much for the code base. Happy to scale them back in any way.