Make sure methods are callable

[ghickman]

From #37:

Django's DeferredAttribute gets reported as method by pydoc.classify_class_attrs, but is not callable, so Method.from_func crashes.

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Now that 61d6119 is in, the tests for this should correctly fail in CI.

[ghickman]

This needs a use case to go with it, and since the original goal was to support Django's DeferredAttributes, a way to exercise that use case in tests.

This is all a bit related to #41, but that's likely too much for this ticket.