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Without seeing the original code, I'm not convinced this is
a Python problem. This is due to the fact that inheriting
from builtin's and standard library objects defined in C
works fine.

For instance, inheriting dict's __len__ is shown here:
>>> class foo(dict): pass
...
>>> len(foo())
0
>>> foo.__len__
<slot wrapper '__len__' of 'dict' objects>

But grep shows the dict source makes no reference to
'__len__'  directly:
[...@localhost src]$ grep -Hc "__len__" Objects/dictobject.c
Objects/dictobject.c:0

The way it works is that PyType_Ready generates the
appropriate __dict__ entries for all of the special methods
that are defined at the C level (i.e. have non-null pointers).

If you aren't invoking PyType_Ready before using your class,
then it won't work properly. (This invocation is usually
made in the C module's initialisation method). See the
"Extending and Embedding" docs for all the gory details.