replace self with cls for class method

CONTRIBUTORS.rst

@@ -196,6 +196,7 @@ Contributors
 * Marco Fucci
 * Mihail Russu
 * Robert Slotboom
+* Erick M'bwana
 
 Translators
 ===========

wagtail/wagtailadmin/edit_handlers.py

@@ -448,8 +448,8 @@ class BaseFieldPanel(EditHandler):
         return mark_safe(render_to_string(self.field_template, context))
 
     @classmethod
-    def required_fields(self):
+    def required_fields(cls):
-        return [self.field_name]
+        return [cls.field_name]
 
 
 class FieldPanel(object):