Issue #15535:  Fix pickling of named tuples.
merge
Issue #15902: Fix imp.load_module() to accept None as a file when
trying to load an extension module.

While at it, also add a proper unittest.skipIf() guard to another test
involving imp.load_dynamic().
#15902: merge w/ 3.3
Guard more tests in test_imp requiring imp.load_dynamic() to exist.
Move test_imp over to unittest.main()
add trailing newline to file