bpo-31975 (PEP 565): Show DeprecationWarning in __main__ by ncoghlan · Pull Request #4458 · python/cpython

- primary change is to add a new default filter entry for
  'default::DeprecationWarning:__main__'
- secondary change is an internal one to cope with plain
  strings in the warning module's internal filter list
  (this avoids the need to create a compiled regex object
  early on during interpreter startup)

While CPython doesn't implicitly import `warnings` at startup,
some other implementations might do so. Explicitly clearing the
module cache ensures the subprocess also gets the pure Python
version, no matter what.