bpo-29620: iterate over a copy of sys.modules (GH-4800) (GH-20816)

bpo-29620: iterate over a copy of sys.modules (GH-4800) (GH-20816)

unittest.TestCase.assertWarns no longer raises a RuntimeException
when accessing a module's ``__warningregistry__`` causes importation of a new
module, or when a new module is imported in another thread.
(cherry picked from commit 46398fba4d66ad342cf2504ef947b5fb857423b2)

Co-authored-by: kernc <kerncece@gmail.com>