Fix #6628 - JSONDecodeError are not deserializable

requests.exceptions.JSONDecodeError are not deserializable: calling
`pickle.dumps` followed by `pickle.loads` will trigger an error.

This is particularly a problem in a process pool, as an attempt to
decode json on an invalid json document will result in the entire
process pool crashing.

This is due to the MRO of the `requests.exceptions.JSONDecodeError`
class: the `__reduce__` method called when pickling an instance is not
the one from the JSON library parent: two out of three args expected
for instantiation will be dropped, and the instance can't be
deserialised.

By specifying in the class which parent `__reduce__` method should be
called, the bug is fixed as all args are carried over in the resulting
pickled bytes.

src/requests/exceptions.py

@@ -41,6 +41,16 @@ class JSONDecodeError(InvalidJSONError, CompatJSONDecodeError):
         CompatJSONDecodeError.__init__(self, *args)
         InvalidJSONError.__init__(self, *self.args, **kwargs)
 
+    def __reduce__(self):
+        """
+        The __reduce__ method called when pickling the object must
+        be the one from the JSONDecodeError (be it json/simplejson)
+        as it expects all the arguments for instantiation, not just
+        one like the IOError, and the MRO would by default call the
+        __reduce__ method from the IOError due to the inheritance order.
+        """
+        return CompatJSONDecodeError.__reduce__(self)
+
 
 class HTTPError(RequestException):
     """An HTTP error occurred."""

tests/test_requests.py

@@ -2810,3 +2810,13 @@ class TestPreparingURLs:
         assert r4 == 425
         assert r5 == 425
         assert r6 == 425
+
+
+def test_json_decode_errors_are_serializable_deserializable():
+    json_decode_error = requests.exceptions.JSONDecodeError(
+        "Extra data",
+        '{"responseCode":["706"],"data":null}{"responseCode":["706"],"data":null}',
+        36,
+    )
+    deserialized_error = pickle.loads(pickle.dumps(json_decode_error))
+    assert repr(json_decode_error) == repr(deserialized_error)