I'm skeptical about the whole revision 53997
which introduced not only the unneeded metaclass
condition, but also the strange check when
assigning to __bases__.  I don't think that this
check about __bases__ is correct or relevant or
really fixes any crash.  The link between the
bases and the metaclass of a class is tenuous
anyway: the bases are just used to compute the
metaclass if none is specified explicitly.

As I said on python-dev (with no answer) I am
thinking about reverting r53997 completely.
I'm going to check what I said above a bit more
in-depth first.