do not read descriptors when iterating class dict

Iterating a class dictionary, e.g. ``Foo.__dict__.keys()``, yields
the descriptor *value* rather than the descriptor itself.  If
reading the descriptor fails, the member is omitted entirely.

Change PythonType.GetMemberDictionary to include the descriptor
object itself instead of trying to read it.
Merge remote-tracking branch 'frasertweedale/fix/dictproxy-descriptor-iteration' into ipy-2.7-maint