- switched to LookupError for the dedicated exception
- default value moved to a CodecInfo class attribute
- new private APIs guarded by PY_LIMITED_API
- used repr formatting where appropriate in the tests
- cleaned up the tests a bit by using encodings.normalize_encoding
- new test to ensure the codec output type checks are still exercised
- backwards compatibility tweaks for raw tuples returned from the codec registry lookup (uncovered by the full test suite run)

I'll be committing this version after a final local run of "make test" and a refleak check on test_codecs, test_charmapcodec and test_unicode (the latter two are the ones that found the backwards compatibility issue with the attribute lookup).