* Add optional uvloop import
- If we find `uvloop` in the env for black, blackd or black-primer lets try and use it
- Add a uvloop extra install
Fixes#2257
Test:
- Add ci job to install black[uvloop] and run a primer run with uvloop
- Only with latest python (3.9)
- Will be handy to compare runtimes as a very unoffical benchmark
* Remove tox install
* Add to CHANGES/news
