browse/utils: Use python-magic when available instead of file-magic

swh-web uses the magic module to detect mime_type and encoding of contents stored
inside the archive. Its is provided by the python-magic package (debian buster)
or by the file-magic package (debian stretch).

As python-magic is more stable than file-magic (see a2759fec), prefer to use
it instead. Nevertheless, keep backward compatibility for file-magic as it is
currently used in production environment.