Importing ABC directly from collections module was removed in Python 3.9

https://bugzilla.redhat.com/show_bug.cgi?id=1792972

https://github.com/chigby/mtg/blob/c12976d6c7e13d6859a0fbac4c057dc9529de473/mtglib/gatherer_request.py#L3

Thanks for the library!