bpo-41003: Fix test_copyreg when numpy is installed (GH-20935) (GH-20945) (GH-20946)

bpo-41003: Fix test_copyreg when numpy is installed (GH-20935) (GH-20945) (GH-20946)

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.

(cherry picked from commit 8362893e3fe083df2ec8bb94c28b1a78383eadbf)
(cherry picked from commit b39d41ba1b77f7bc51c4d6f6d0e336693192cb3a)