…keyring queried for credentials.

Since there is currently no possible way to get pip to query the keyring when --no-input is used I am not marking the new scenario xfail. That should change once there is some way to do that.

Currently, Pip is conservative and does not query keyring at all when `--no-input` is used because the keyring might require user interaction such as prompting the user on the console.

This commit adds a flag to force keyring usage if it is installed. It defaults to `False`, making this opt-in behaviour.

Tools such as Pipx and Pipenv use Pip and have their own progress indicator that hides output from the subprocess Pip runs in. They should pass `--no-input` in my opinion, but Pip should provide some mechanism to still query the keyring in that case. Just not by default. So here we are.

…r_keyring_if_needed to add a few --no-input and --force-keyring scenarios

…-defined]` comments that are no longer needed as of pytest 7.1.3