The following code causes warn_explicit() (in Python/_warnings.c) to raise a
SystemError:

import warnings
warnings.filterwarnings('once')
warnings.onceregistry = None
warnings.warn_explicit(message='foo', category=Warning, filename='bar',
                       lineno=1, registry=None)


this is because warn_explicit() assumes that warnings.onceregistry is a dict,
and passes it to update_registry(), which passes it to already_warned(), which
eventually passes it to _PyDict_SetItemId(), which raises the SystemError.