Don't use python's collections.Set (#7875)

Don't use python's collections.Set (#7875)

We used to use collections.Set.

Apparently this was an alias for collections.abc.Set. I can't find it
documented anywhere though, going as far back as Python 3.5. Perhaps it
was an undocumented left-over from when the collections.abc module was
split out from collections in Python 3.3?

At any rate, the alias appears broken in Python version 3.10.1.

Now we just use the builtin set type, which is what we wanted anyway,
I think.