Fix test_bad_address on Ubuntu 13.04
Issue #13612: handle unknown encodings without a buffer overflow.

This affects pyexpat and _elementtree. PyExpat_CAPI now exposes a new
function - DefaultUnknownEncodingHandler.

Based on a patch by Serhiy Storchaka.
Clean-up duplicated code in tests
Various tweaks to importlib docs.
Add a missing parenthesis.
Mention __cached__ in the import ref.
Issue #18063: fix some struct specifications in the tests for sys.getsizeof().