Make sure to use a Python 3 pylint

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>
diff --git a/tests/scripts/check-python-files.sh b/tests/scripts/check-python-files.sh
index 6b864d2..cd18518 100755
--- a/tests/scripts/check-python-files.sh
+++ b/tests/scripts/check-python-files.sh
@@ -9,15 +9,10 @@
 # Run 'pylint' on Python files for programming errors and helps enforcing
 # PEP8 coding standards.
 
-# Find the installed version of Pylint. Installed as a distro package this can
-# be pylint3 and as a PEP egg, pylint. We prefer pylint over pylint3
-if type pylint >/dev/null 2>/dev/null; then
-    PYLINT=pylint
-elif type pylint3 >/dev/null 2>/dev/null; then
-    PYLINT=pylint3
+if type python3 >/dev/null 2>/dev/null; then
+    PYTHON=python3
 else
-    echo 'Pylint was not found.'
-    exit 1
+    PYTHON=python
 fi
 
-$PYLINT -j 2 scripts/*.py tests/scripts/*.py
+$PYTHON -m pylint -j 2 scripts/*.py tests/scripts/*.py