Draft: Fixup for Python 3.10
Closes issue #109. Long story short: a few names from collection are
now moved to collection.abc exclusively starting in Python 3.10. The
only name this app uses from there that was moved is
collections.Iterable
. Python versions starting from 3.3 support both
collections.Iterable
and collections.abc.Iterable
as the way to refer to
this class, which Python 3.10 being the first one to drop
collections.Iterable
. So.. we just work around this API quirk
and always refer ot it as collections.abc.Iterable
.