consider validating that all tilde-notation entries have 3 places

[birkin]

The concern is that if the tilde-notation is incorrect, and only goes to, say, two-places, that a full minor-release upgrade could be triggered.

That would conflict with the intentionally-conservative goal of this app: to only auto-update the "patch" version (in the context of (major/minor/patch syntax) of an entry in a requirements.in file.