This is the plugins manager counterpart to openstenoproject/plover#1601. These repositories are mutually dependent so both PRs should be merged together. For CI purposes I've pointed the pip dependencies at each other (i.e. at my own branches), this will need changing back before merging.

The changes needed here are pretty representative of the changes any plugin would need to be PyQt6 compatible. I might make a migration guide if people are interested.