The docs for NullImporter need to not mention that it is used with sys.path_importer_cache since it isn't. As for sys.path_importer_cache, it should say that None is inserted and not NullImporter. Both need a versionchanged notation of the switch from None to NullImporter, but sys.path_importer_cache needs a special mention that to be fully backwards-compatible to check for both None and NullImporter.

And just to make sure you are looking at the right docs, Andreas, your links were to the Python 2.7 docs. If you look at the top of the docs you will see a dropdown which lets you switch to the 3.3 or 3.4 docs easily.