scripts: check_compliance: fix filter_py edge case

The filter_py() function is handed a bunch of file names and is
expected to give back just the python files.

Its input is the output from a 'git diff' command which normally when
used in the vanilla zephyr repository just prints regular files.

In situations where we're checking compliance on a repository with
submodules, however, filter_py() can be given directories in the
'fnames' list from the git output.

In the interests of being defensive and sharing infrastructure, just
handle this case in filter_py().

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>