Windows bytes API is deprecated

[jaraco]

With the recent introduction of builds on newer versions of visual studio (#1904), there are new deprecation warnings in the Appveyor tests like this:

self = <setuptools.sandbox.DirectorySandbox object at 0x00000224C2581F60>
path = b'MANIFEST.in', args = (), kw = {}
    def wrap(self, path, *args, **kw):
        if self._active:
            path = self._remap_input(name, path, *args, **kw)
>       return original(path, *args, **kw)
E       DeprecationWarning: The Windows bytes API has been deprecated, use Unicode filenames instead

These should be fixed.