The current code is:
    ...
    if os.path.dirname(cmd):
        if _access_check(cmd, mode):
            return cmd
        return None
    ...

In Windows, if 'cmd' include directory path and not include extension name, it return 'None'.
e.g. a file's path is 'd:\dir\app.exe', call shutil.which with 'cmd=="d:\dir\app"'.

How about this patch:
    ...
    if os.path.dirname(cmd):
        path, cmd = os.path.split(cmd)
    ...