This seems to be a useful feature to me.  Another area where it can help
is with fixing function types.  Currently the qualname can be lost:

>>> def f():
...    def g():
...       pass
...    return g
... 
>>> g = f()
>>> g
<function f.<locals>.g at 0x7f1dac4d8ba0>
>>> types.FunctionType(f.__code__, {})
<function f at 0x7f1dac4dfae0>
>>> types.FunctionType(g.__code__, {})

There is no way to specify a qualname when constructing a
'FunctionType'.  The name is derived from the code object.  The
qualname could be too if we fix this issue.  I will open another issue
for the 'FunctionType' problem.

I have attached a WIP patch.  It passes the full test suite.  I am 
somewhat unsure about the CAPI changes, though.  They are a breaking API
change.  Do we need a *_QualName version instead like we did for
'PyFunction_NewWithQualName'?  I think probably so.  I will add doc
updates to the patch once the API stuff gets worked out.