Travis: run Pylint

Declare an explicit Python version. Pick 3.5 which is the default
version on Ubuntu 16.04. This is necessary on Travis to have a working
pip for Python 3.

Install Pylint 2.4.4. There's nothing special about this version, it's
just the latest version.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

.travis.yml

@@ -11,6 +11,10 @@ jobs:
           packages:
           - doxygen
           - graphviz
+      language: python # Needed to get pip for Python 3
+      python: 3.5 # version from Ubuntu 16.04
+      install:
+        - pip install pylint==2.4.4
       script:
         - tests/scripts/recursion.pl library/*.c
         - tests/scripts/check-generated-files.sh
@@ -18,6 +22,7 @@ jobs:
         - tests/scripts/check-names.sh
         - tests/scripts/check-files.py
         - tests/scripts/doxygen.sh
+        - tests/scripts/check-python-files.sh
 
     - name: default configuration
       script: