PEP-0561 does not mention extension packages, packages that consist of just an .so file. Given that PEP-484 mentions extension modules as a use case for stub files, should PEP-561 make it more clear what the ideal layout of an extension only package is?
It appears to be valid to have a package structure like the following:

.../site-packages/
    mypackage/
        __init__.pyi
        py.typed
    mypackage.cpython-37m-x86_64-linux-gnu.so