I think there are a couple ways we can go about this.  It looks like
extensions just specify tp_dictoffset if they want their instances
to have instance attributes, but don't usually actually look at the
dict or really make use of the fact that it's a real dict.  So possibly,
we have the ability to stuff a non-dict into the slot they reserved for
us.

Seems risky though.  Instead, the approach in this commit is to create
an actual dict and put it in like they expect.  All the attribute-accessing
paths have been updated to look at both the fast Pyston HCAttrs method, and
the CAPI dict method.