with_metaclass: check existence of `__prepare__` attribute before calling it

[behnam]

plugin-python/vendor/python/six.py

Line 831 in 04dc40a

return meta.__prepare__(name, bases)

The def with_metaclass() implementation assumes that all meta classes have __prepare__ defined.

The Python Reference says (https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace):

If the metaclass has a __prepare__ attribute, it is called as [...].

If the metaclass has no __prepare__ attribute, then the class namespace is initialised as an empty ordered mapping.

Therefore, I think six should check existence of __prepare__ attribute before calling it, as __prepare__ is not defined on type in PY2.

[behnam]

Oops, wrong repo. It's here: benjaminp/six#252