swh.lister.gnu: Add function to check for file extension.

Added a function which will derive the extension from filename
and check if the fie extension match the type of file that is to be
archived.

README.md

@@ -13,10 +13,10 @@ following Python modules:
 - `swh.lister.debian`
 - `swh.lister.github`
 - `swh.lister.gitlab`
+- `swh.lister.gnu`
 - `swh.lister.pypi`
 - `swh.lister.npm`
 - `swh.lister.phabricator`
-- `swh.lister.gnu`
 
 Dependencies
 ------------