Let devpi fall back to other options if keyring fails
Let devpi fall back to other options if keyring fails

keyring provides a plugin to supply the password to devpi-client. However, if keyring fails - e.g. it's installed but no backend is working, or unlocking the keyring fails - then `devpi login` becomes impossible, because the keyring exception is uncaught.

This catches any KeyringError, and returns None instead, which signals to devpi that it should try another way to get the password ([docs](https://devpi.net/docs/devpi/devpi/stable/+d/devguide/hooks.html#devpi-client-plugin-hooks-experimental)).
jaraco/keyring