Handle python updates

[antistic]

Recently python updated to 3.10. This caused a bunch of plover plugins to seemingly disappear, since the old python3.9 user site directory is no longer in the path.

It might be useful to have a warning message about this and/or ask to automatically install missing plugins.

[user202729]

(Plover is a Python package as well. How can it survive the update?)

I guess the plugin can store a list of installed plugins separately from pip, then list the disappeared ones as "recently uninstalled plugins".