The idea in this PR is to converge on Python's `fsdecode` and `fsencode`
for argument manipulation. This seems to match up with the Python
standard library's assumptions: namely, on Windows, we use `fsdecode` to
get back to Unicode strings:
https://github.com/python/cpython/blob/54bbb5e3363ef3634f1fd7521ba3f3c42c81a331/Lib/subprocess.py#L561

So let's start by dropping this utility and going straight for
`fsdecode` here to match.

This can apparently never be `None`, as of Python 3.2.

On Windows, converting command-line arguments (hopefully!!!) only needs
to deal with valid strings from the OS. So it is not really relevant to
test with non-UTF-8, non-surrogate bytes.