Reworked for the Python-3-only world.
Added:
* Regular prefix-based commands for `.flip` and `.roll` (9)
Changed:
* Rolling always uses the "down a hill" format that was previously an Easter
egg, to differentiate it more from flipping (8)
Meta:
* Package metadata modernized using `pyproject.toml` (8)
* Refactored a big chunk of the plugin code (8)
* Python older than 3.8 is no longer officially supported (8)
* Sopel 7.1 bots are still supported, if running on Python >=3.8
* Releases are now automated (8)