bpo-43693: Silence some compiler warnings. (gh-26588)

The plan is to eventually make PyCodeObject opaque in the public C-API, with the full struct moved to Include/internal/pycore_code.h. _PyLocalsPlusKinds and _PyLocalsPlusKind started off there but were needed on PyCodeObject, hence the duplication. This led to warnings with some compilers. (Apparently it does not trigger a warning on my install of GCC.)

This change eliminates the superfluous typedef.

https://bugs.python.org/issue43693