Don't look up other modules objects in __del__ methods

Don't look up other modules objects in __del__ methods

The modules might have been already cleaned up during interpreted
shutdown. See <https://bugs.python.org/issue46256#msg409847> for an
explanation.

The stdlib guards against the same thing happening too. However they
take a reference in the function closure, which is stronger than what we
do. Doing so, on our strictly typed codebase, is a tedious chore, so, if
this is enough (it should be, according to the OP), I'm happy this way.

Close #198.