Fix coverage

Author: Cito

src/graphql/pyutils/description.py

@@ -51,7 +51,7 @@ def unregister(cls, base: type) -> None:
             msg = "Only types can be unregistered."
             raise TypeError(msg)
         if isinstance(cls.bases, tuple):
-            if base in cls.bases:
+            if base in cls.bases:  # pragma: no branch
                 cls.bases = tuple(b for b in cls.bases if b is not base)
             if not cls.bases:
                 cls.bases = object