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I debugged this a bit by comparing the behavior of 3.3 against 3.2. For both cases I used the following code and debugged it in Python via pdb*:
 import importlib
 importlib.__import__('my_test_package')
ISTM that the difference in behavior is a result of what loader gets chosen for the initial `import 'my_test_package'`.
With 3.2 a importlib._bootstrap._SourceFileLoader loader gets created against 'my_test_package/__init__.py'. This works fine because _SourceFileLoader fixes up sys.modules when it loads.
With 3.3 a _frozen_importlib.ExtensionFileLoader loader gets created against 'my_test_package/__init__.so'. This doesn't work because ExtensionFileLoader does *not* fixup sys.module when it loads.
I hope that helps some.
* Which was a real pain for 3.3 since you are debugging the frozen importlib. You get line numbers at least, but it would be really nice if you could tell pdb a source file to use when you are dealing with bytecode only objects and\or you could disassemble the bytecode from pdb.