Provide a nicer way to disable keyring

[jaraco]

Historically, the recommended way to disable keyring was to uninstall it... but as revealed in pypa/twine#338, it's not always possible for a user who has keyring installed by a system package manager to uninstall a package without also uninstalling or invalidating other packages.

So let's provide a simple mechanism that disables keyring. Here's what I have in mind:

• Provide a Null Keyring backend that returns None for all get operations.
• Provide a command-line that configures the user's environment for this Null keyring. Something like keyring --disable.
• For one-off invocations, also honor an environment variable like DISABLE_PYTHON_KEYRING that if present will give priority to the Null keyring.
• For one-off invocations, honor an environment variable PYTHON_KEYRING_BACKEND to select a particular keyring such as the Null keyring.

[jaraco]

Released as 15.1.0.