Fix a typing warning from mypy involving httpx (#125)

Co-authored-by: Casey <casey.duquette@snapchat.com>

gidgethub/httpx.py

@@ -16,13 +16,7 @@ async def _request(
     ) -> Tuple[int, Mapping[str, str], bytes]:
         """Make an HTTP request."""
         response = await self._client.request(
-            method,
-            url,
-            # Reason: see discussion at https://github.com/brettcannon/gidgethub/pull/122#issuecomment-633738024.
-            # httpx 0.13.1 updated the HeaderTypes and it is acting invariant, causing our Mapping[str, str] to be
-            # technically incompatible with their Mapping[Union[str, bytes], Union[str, bytes]].
-            headers=dict(headers),  # type: ignore
-            data=body,
+            method, url, headers=dict(headers), data=body
         )
         return response.status_code, response.headers, response.content