Software update checks

[bartfeenstra]

Definition of Done

• Deploy an authoritative list of all Betty releases
• Have Betty, if it is run as a packaged release, periodically check this list for updates
• Notify users of updates in the CLI. Can we do this asynchronously, yet make sure the output does not get lost among all the other async output? Log this as a warning so it shows up by default.
• Add a flag to the CLI to prevent update checks.
• Use separate messages for minor updates and major upgrades

Proposals

• Query the PyPI JSON API. Use the packaging package for version parsing.
• Fetch and cache the API response