`constants_from_enum` does not work for `StrEnum`

`constants_from_enum` does not work for `StrEnum`.

Here is an example of an error caused by this bug:
```
Traceback (most recent call last):
...
gin.constants_from_enum(Language)
File "/workspace/finch/.venv/lib/python3.11/site-packages/gin/config.py", line 2798, in constants_from_enum
return decorator(cls)
^^^^^^^^^^^^^^
File "/workspace/finch/.venv/lib/python3.11/site-packages/gin/config.py", line 2793, in decorator
constant('{}.{}'.format(module, str(value)), value)
File "/workspace/finch/.venv/lib/python3.11/site-packages/gin/config.py", line 2756, in constant
raise ValueError("Invalid constant selector '{}'.".format(name))
ValueError: Invalid constant selector 'pipecat.transcriptions.language.af-ZA'.
```