[Python] Fix extension implementation lookup
[Python] Fix extension implementation lookup

The wrong implementation could be returned if an error happened in
PyObject_IsSubclass() (for instance because a gtype wrapper was not
available).

gregier/libpeas,gregier/libpeas,Distrotech/libpeas,GNOME/libpeas,gregier/libpeas,chergert/libpeas,Distrotech/libpeas,chergert/libpeas,chergert/libpeas,GNOME/libpeas,Distrotech/libpeas,gregier/libpeas