[FEATURE] Add "Latest" non-failure testing to (at least) the main branch

[teald]

Is your feature request related to a problem? Please describe or write "N/A".

This is to prevent situations like Issue #68 / PR #69 from happening n the future, and to catch important dependency upgrades that need to be reacted to.

Describe the solution you'd like

Ideally, a run of all the tests using a freshly-updated lockfile instead of the latest development one in the main repo. This should be as simple as running poetry lock on the Actions repo clone.

Notes

• This shouldn't cause PRs to fail, if it's tested with them
• This shouldn't prevent releases, as problems with installation and dependency resolution are already handled in build testing
  • Any future issues with builds should also go into build testing, not be a part of these tests
• This shouldn't replace or extend any extant tests
• Should be lowest-priority
• Should be added to cron tests