Style check: adapt to changes in container image

The container image used in CI for the static checks now has black
installed from pip.  This means that the executable "black" may not be
always accessible, but using "python3 -m" as an entrypoint is.

This is a partial backport of 77ca585.

Signed-off-by: Cleber Rosa <[email protected]>

Author: clebergnu

selftests/style.sh

@@ -1,4 +1,4 @@
 #!/bin/sh -e
 echo "** Running black..."
 
-black --check --diff --color .
+python3 -m black --check --diff --color .