Fix code to match cypthon expectation

It fails to convert unicode to 'str', we have to help it out.
```
Error compiling Cython file:
------------------------------------------------------------
...
    if metadata_request_timeout is None:
        return stmt
    ms = int(metadata_request_timeout / datetime.timedelta(milliseconds=1))
    if ms == 0:
        return stmt
    return f"{stmt} USING TIMEOUT {ms}ms"
           ^
------------------------------------------------------------

cassandra/util.py:1813:11: Cannot convert Unicode string to 'str' implicitly. This is not portable and requires explicit encoding.
```

Author: dkropachev

Diff for: cassandra/util.py

@@ -1810,4 +1810,4 @@ def maybe_add_timeout_to_query(stmt: str, metadata_request_timeout: Optional[dat
     ms = int(metadata_request_timeout / datetime.timedelta(milliseconds=1))
     if ms == 0:
         return stmt
-    return f"{stmt} USING TIMEOUT {ms}ms"
+    return str(f"{stmt} USING TIMEOUT {ms}ms")