atomic get_or_create

I'd like to be able to do something like:

```python
# mysite/settings.py
SECRET_KEY = keyring.get_or_create_password(__package__, "secret_key")
```

and have it work in a variety of adverse scenarios - eg gunicorn threaded, multiprocessing workers. (although in this situation I currently set an environment variable)

No repository file or test is named. Start by tracing the requested get_or_create_password API from the Python example, then examine how it should behave with gunicorn threaded and multiprocessing workers. Done means the operation is atomic across those adverse scenarios, with tests covering the stated cases.

Written by the indexing model from the issue text.