Makefile.am: add .in files to fastlint target
    
    Previously fastlint would only process .py files.
    Detect which .in files are in fact Python files and add
    them to the list of files to process during the fastlint
    Makefile target.
    Original change suggested by Alexander Bokovoy.
    
    Fixes: https://pagure.io/freeipa/issue/7984
    Signed-off-by: François Cami <fcami@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>