bpo-41003: Fix test_copyreg when numpy is installed (GH-20935)

Fix test_copyreg when numpy is installed: test.pickletester now
saves/restores warnings.filters when importing numpy, to ignore
filters installed by numpy.

Add the save_restore_warnings_filters() function to the
test.support.warnings_helper module.

tests/__init__.py

@@ -15,26 +15,25 @@ by import rather than matching pre-defined names.
 import os
 import sys
 import unittest
-import warnings
 from test.support import run_unittest
+from test.support.warnings_helper import save_restore_warnings_filters
 
 
 here = os.path.dirname(__file__) or os.curdir
 
 
 def test_suite():
-    old_filters = warnings.filters[:]
     suite = unittest.TestSuite()
     for fn in os.listdir(here):
         if fn.startswith("test") and fn.endswith(".py"):
             modname = "distutils.tests." + fn[:-3]
-            __import__(modname)
+            # bpo-40055: Save/restore warnings filters to leave them unchanged.
+            # Importing tests imports docutils which imports pkg_resources
+            # which adds a warnings filter.
+            with save_restore_warnings_filters():
+                __import__(modname)
             module = sys.modules[modname]
             suite.addTest(module.test_suite())
-    # bpo-40055: Save/restore warnings filters to leave them unchanged.
-    # Importing tests imports docutils which imports pkg_resources which adds a
-    # warnings filter.
-    warnings.filters[:] = old_filters
     return suite