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The new importlib shows a regression w.r.t. previous CPython versions. It no longer recognises an "__init__.so" file as a package. All previous CPython versions have always tested first for an extension file before testing for a .py/.pyc file. The new importlib explicitly excludes the ExtensionFileLoader from loading packages. See importlib/_bootstrap.py, line 1579 onwards:
 1579 def _get_supported_file_loaders():
 1580 """Returns a list of file-based module loaders.
 1581 
 1582 Each item is a tuple (loader, suffixes, allow_packages).
 1583 """
 1584 extensions = ExtensionFileLoader, _imp.extension_suffixes(), False # <== bug here
 1585 source = SourceFileLoader, SOURCE_SUFFIXES, True
 1586 bytecode = SourcelessFileLoader, BYTECODE_SUFFIXES, True
 1587 return [extensions, source, bytecode]
(BTW, I'm not sure what to file this bug against - "Interpreter Core" or "Library"?)