[Feature request] Python code formatting option using black

[Minyus]

Thank you for this awesome tool!
For Python development, Python code formatting (using black) is a common practice to:

• ensure readability
• detect Python syntax error before running
• save time for manual cosmetic formatting
• keep the Git history clean (avoid functional and cosmetic modifications mixed in each commit)
• keep each line short (88 characters per line using black in default)

It would be great if pyvim supports an option to run Python code formatting (upon saving by :w)