Default enum description to "An enumeration."

default to this string, which is used in many tests, is causing

graphene/types/enum.py

@@ -63,7 +63,7 @@ class EnumMeta(SubclassWithMeta_Meta):
         cls, enum, name=None, description=None, deprecation_reason=None
     ):  # noqa: N805
         name = name or enum.__name__
-        description = description or enum.__doc__
+        description = description or "An enumeration."
         meta_dict = {
             "enum": enum,
             "description": description,