bpo-36724: Add _PyWarnings_Fini() (#12963)

Py_FinalizeEx() now clears _PyRuntime.warnings variables and
_PyRuntime.exitfuncs.

Changes:

* Add _PyWarnings_Fini(): called by Py_FinalizeEx()
* call_ll_exitfuncs() now clears _PyRuntime.exitfuncs while iterating
  on it (on backward order).