Don't hard-code class names when calling static methods

normalize_email should be called on the instance, not the class. This
has the same effect normally but is more helpful to subclassers. When
methods are called directly on the class, subclasses can't override
them.

docs/topics/auth/customizing.txt

@@ -939,7 +939,7 @@ authentication app::
                 raise ValueError('Users must have an email address')
 
             user = self.model(
-                email=MyUserManager.normalize_email(email),
+                email=self.normalize_email(email),
                 date_of_birth=date_of_birth,
             )