Remove some Python 3.10 features to make the add-on compatible with py39.
This is the Python version that's bundled with Blender 2.93 LTS, for which
I got a request to see if it could be supported.
The Blender version still isn't officially supported, but this should make
things at least not immediately fail.
Instead of storing the cached manager info in the Blender preferences,
store the info in a JSON file. The file is located in the user prefs
folder (`~/.config/blender/{version}/config` on Linux).
This also reduces the number of 'refresh' operators to a single one, which
then fetches all necessary info from the Manager.
This fixes an issue (reported via chat) where worker tags were sometimes
not retained across file saves.
