X allow `cdef class Klass(bytes)`

TODO check that no fields are added and then we can safely allow
inheritance from PyVarObject.

See https://github.com/cython/cython/issues/711 for details.

Cython/Compiler/Nodes.py

@@ -4756,7 +4756,7 @@ class CClassDefNode(ClassDefNode):
                      base_type.is_final_type:
                 error(base.pos, "Base class '%s' of type '%s' is final" % (
                     base_type, self.class_name))
-            elif base_type.is_builtin_type and \
+            elif False and base_type.is_builtin_type and \
                      base_type.name in ('tuple', 'str', 'bytes'):
                 error(base.pos, "inheritance from PyVarObject types like '%s' is not currently supported"
                       % base_type.name)