Windows bytes API is deprecated

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.

Start by reproducing the warning in the AppVeyor tests and tracing the setuptools.sandbox.DirectorySandbox.wrap call shown in the issue. Search the sandbox implementation for the path remapping and Windows filesystem calls involved with b'MANIFEST.in'. Done means the deprecation warning is gone from the Windows test run while existing sandbox behavior remains intact.