fix: decode bytes to produce a string

With the new JSON library, ``dumps()`` seems to return bytes.
Here we decode the output of ``dumps()``, ensuring the output is
converted into text.
This effectively should address the issue linked below.

Resolves: #1352.

mriqc/interfaces/webapi.py

@@ -163,8 +163,8 @@ def _run_interface(self, runtime):
                 | orjson.OPT_APPEND_NEWLINE
                 | orjson.OPT_SERIALIZE_NUMPY
             ),
-        )
-        Path('payload.json').write_bytes(payload_str)
+        ).decode("utf-8")
+        Path('payload.json').write_text(payload_str)
         self._results['payload_file'] = str(Path('payload.json').absolute())
 
         try: