Make HttpStatusError and RequestError pickleable

If an error has keyword-only arguments, it needs this extra `__reduce__` method
to ensure that unpickling doesn't raise
`TypeError: missing (n) required keyword-only argument(s)`

httpx/_exceptions.py

@@ -71,6 +71,13 @@ def request(self) -> Request:
     def request(self, request: Request) -> None:
         self._request = request
 
+    def __reduce__(
+        self,
+    ) -> typing.Tuple[
+        typing.Callable[..., Exception], typing.Tuple[typing.Any], dict[str, typing.Any]
+    ]:
+        return (Exception.__new__, (type(self),) + self.args, self.__dict__)
+
 
 class RequestError(HTTPError):
     """

tests/test_pickle.py

@@ -0,0 +1,16 @@
+import pickle
+
+from httpx import HTTPStatusError, RequestError
+
+
+def test_pickle():
+    req_err = RequestError("hi!", request="request")  # type:ignore[arg-type]
+    req_err_clone = pickle.loads(pickle.dumps(req_err))
+    assert req_err.args == req_err_clone.args
+    assert req_err.request == req_err_clone.request
+
+    status_err = HTTPStatusError("hi", request="request", response="response")  # type:ignore[arg-type]
+    status_err_clone = pickle.loads(pickle.dumps(status_err))
+    assert status_err.args == status_err_clone.args
+    assert status_err.request == status_err_clone.request
+    assert status_err.response == status_err_clone.response