CompatHelper: bump compat for "PrettyTables" to "3.1"

[github-actions[bot]]

This pull request changes the compat entry for the PrettyTables package from 2.0 to 2.0, 3.1.

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.