On some systems, such as Ubuntu up to 19.04, `pylint` is for Python 2
and `pylint3` is for Python 3, so we should not use `pylint` even if
it's available.
Use the Python module instead of the trivial shell wrapper. This way
we can make sure to use the correct Python version.
Fix#3111
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>