`keyring` backend for `kbs2`

[woodruffw]

The keyring package in Python is a popular way to access a system secret manager. In particular, tools like twine use it.

keyring supports custom backends: https://github.com/jaraco/keyring#third-party-backends. It would be nice to have a kbs2 backend, allowing tools that are aware of keyring to integrate with kbs2 without needing extra accomodations.

This might belong in a separate repository, but tracking here for now.

Ref: pypa/twine#913