cpython_frontend: Support for ~/.virtualenvs directory on unix.

[Gadgetoid]

This is a first gentle step for #2974 to acknowledge that "~/.virtualenvs" is a standard directory where we might find python binaries managed by virtualenvwrapper and add support for it into Thonny.

It adds a simple glob expansion of the "~/.virtualenvs" directory to find python binaries managed by virtualenvwrapper.

The "allvirtualenv" function backing "lsvirtualenv" for virtualenvwrapper actually activates each virtual environment in a sub shell to enumerate them and find their "prompt" or "name". I don't think we need to go this far.

This mirrors VSCode's Python extension, which scans this directory for interpreters.

This is an attempt at - in light of pep688 finding its way into distros - cohesion between VScode, Thonny and user (command-line) Python environments with the least amount of friction.

I plan to use "~/.virtualenvs" as the default home for a "pimoroni" virtual environment on Raspberry Pi OS. This is where our software installers (simple helper scripts for users who don't want to care about Python's idiosyncrasies) will install our Python libraries.

It might also be worth checking the WORKON_HOME environment variable, since VSCode also supports this and the default virtualenvwrapper directory can be overridden by the user. There is no precedent I could find for grabbing an environment variable in cpython_frontend/cp_front.py however.

[aivarannamaa]

I think it's worth consulting WORKON_HOME. If my commit looks right to you then I'll merge it and it will be part of Thonny 5.0.0b1

[Gadgetoid]

This - famous last words - looks reasonable. Thank you!