Set low priority for rm-ing refs section so refs still resolve correctly (#2227)

pep2html.py

@@ -438,8 +438,8 @@ class PEPHeaders(Transform):
 class PEPFooter(Transform):
     """Remove the References section if it is empty when rendered."""
 
-    # Uses same priority as docutils.transforms.TargetNotes
-    default_priority = 520
+    # Set low priority so ref targets aren't removed before they are needed
+    default_priority = 999
 
     def apply(self):
         pep_source_path = Path(self.document['source'])