Return NotImplemented sooner for ErrorDetail equality test (#7531)

The test suite raises warnings when tested against Python 3.9
`DeprecationWarning: NotImplemented should not be used in a boolean context`

Where `r` returns `NotImplemented` then this change returns `NotImplemented` first to avoid the comparison test.

rest_framework/exceptions.py

@@ -73,6 +73,8 @@ class ErrorDetail(str):
 
     def __eq__(self, other):
         r = super().__eq__(other)
+        if r is NotImplemented:
+            return NotImplemented
         try:
             return r and self.code == other.code
         except AttributeError: