CompatHelper: bump compat for PrettyTables to 3, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the PrettyTables package from 0.12, 1.0, 2 to 0.12, 1.0, 2, 3. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[tpapp]

This will be fixed by #68 (not closing since CompatHelper will just reopen).

[tpapp]

Closing in favor of #68.