Set __warningregistry__ to None.

>>> import warnings
>>> __warningregistry__ = None
>>> for i in range(3):
...    warnings.warn("oh noes!")

    

Warning (from warnings module):
  File "<pyshell#3>", line 2
UserWarning: oh noes!

Warning (from warnings module):
  File "<pyshell#3>", line 2
UserWarning: oh noes!

Warning (from warnings module):
  File "<pyshell#3>", line 2
UserWarning: oh noes!