`json` package is not imported in `release.ipynb`

This notebook contains a function:

```
def _load_json(cfg, k):
try: return json.loads(cfg[k])
except json.JSONDecodeError as e: raise Exception(f"Key: `{k}` in .ini file is not a valid JSON string: {e}")
```

However, `json` is not imported before that.
Error will be produced as:
```
raceback (most recent call last):
File "/users/kangl/miniconda3/envs/rapids-22.10/lib/python3.9/site-packages/nbdev/release.py", line 40, in _load_json
try: return json.loads(cfg[k])
NameError: name 'json' is not defined
```

I am willing to make a pull request to fix it if you want.