Style check based on black

Avocado has been using black to enforce a code style for a long time
now, with very good results.  Let's expand the same to Avocado-VT.
With a common code style across Avocado and Avocado-VT it'll be easier
to move libraries to the common autils repo.

Reference: avocado-framework/aautils#3
Signed-off-by: Cleber Rosa <[email protected]>