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`.