(See Include/internal/pycore_warnings.h and Python/_warnings.c.)

The warnings module's state (filters, default action, etc.) is currently stored at the level of the global runtime.  That's a problem for the following reasons:

* Python objects are getting stored in _PyRuntimeState
* it breaks the isolation of behavior between interpreters
* objects are leaking between interpreters
* importing the module in a subinterpreter effectively resets the module's state

While those are all a problem in a future where interpreters don't share the GIL, that last one is a problem right now for people using subinterpreters.

One of the following should happen:

* move warnings state down to PyInterpreterState
* move warnings state into PyInterpreterState.dict
* use the module-state API (PEP 3121)
* just work out of the module's __dict__

I could also see use cases for *also* configuring warnings process-wide but that could be handled separately if actually desired.