Fixed #23923 -- Promoted Django's deprecation warnings to errors in runtests.py
Fixed #17890 -- Added an extra_context parameter to AdminSite.password_change().
Fixed #23930 -- Added copies of captured_std* managers from CPython's test.support.

StringIO import was adapted for compatibility with Python 2.
Fixed #23933 -- Made override_settings(DATABASE_ROUTERS) affect the master router.
Replaced router.routers usage with override_settings(DATABASE_ROUTERS).
Corrected a docstring in tests/migrations/test_operations.py.
Removed a no-op statement in tests/multiple_database/tests.py.
Removed an incorrect docstring in tests/multiple_database/tests.py.