Fix handling of mismatched Python version in virtualenv

[sbivol]

Description

This fixes #2547 for the (default) case when g:black_use_virtualenv = 1. The fix for g:black_use_virtualenv = 0 is a superset of these changes but is more invasive so I'll create a separate branch for it.

To trigger the bug on Linux:

• make sure you don't have black installed as a system package: apt purge black
• launch vim built with Python 3.X
• run :BlackVersion, let it finish creating the virtualenv, close vim
• launch vim built with Python 3.Y
• run :BlackVersion, you should see an error: ModuleNotFoundError: No module named 'black'

I usually encounter this issue when I upgrade to a new Kubuntu release which has a newer Vim built against a newer Python, since I keep the same user data so the old virtualenv created for black is still there.

Checklist - did you ...

• [x] Add an entry in CHANGES.md if necessary?
• [x] Add / update tests if necessary?
• [x] Add new / update outdated documentation?