Use a more generic mechanism

[pganssle]

This currently is tightly bound to the variants library specifically, which means that if users vendor variants, they'll also likely need to vendor sphinx_autodoc_variants as well, possibly messing with some code paths at the same time.

This will also fail for things like dateutil.gettz, which is a callable that has other member functions as well. It might be worth using some duck-typing solution to this problem.