Catch utf8 decoding errors explicitly.

Previously, utf8 decode errors bubbled up into json.JSONDecodeError error
handling, which triggered a bug in py310 specifically:

- The failing test was test_http_multipart_invalid_utf8 (test #17)
- It sends invalid utf8 bytes (\xff\xfe) in the multipart response
- aiohttp's part.text() raises a UnicodeDecodeError
- The HTTP multipart transport wasn't catching UnicodeDecodeError, only json.JSONDecodeError
- When pytest tried to format the uncaught underlying UnicodeDecodeError, the exceptiongroup package in py310 hit an internal bug:

    File "/home/runner/work/gql/gql/.tox/pypy3/lib/pypy3.10/site-packages/exceptiongroup/_formatting.py", line 329, in format_exception_only
        yield _format_final_exc_line(stype, self._str)
    File "/home/runner/work/gql/gql/.tox/pypy3/lib/pypy3.10/site-packages/exceptiongroup/_formatting.py", line 33, in _format_final_exc_line
        line = f"{etype}: {valuestr}\n"
    SystemError: unexpected internal exception (please report a bug): <CheckError object at 0x336b7e90>

Catching the utf8 decoding error specifically does actually improve
error handling anyway since it gives a more specific warning to the
user imo.

Author: magicmark

gql/transport/http_multipart_transport.py

@@ -303,10 +303,11 @@ async def _parse_multipart_part(
                 errors=payload.get("errors"),
                 extensions=payload.get("extensions"),
             )
+        except UnicodeDecodeError as e:
+            log.warning(f"Skipping part - failed to decode as UTF-8: {e}")
+            return None
         except json.JSONDecodeError as e:
-            log.warning(
-                f"Failed to parse JSON: {e}, body: {body[:100] if body else ''}"
-            )
+            log.warning(f"Skipping part - failed to parse JSON: {e}")
             return None
 
     async def execute(