Added PEP8 check to GitLab CI.

.gitlab-ci.yml

+before_script:
+  - pip install pep8
+
+pep8:
+  script:
+    # Run pep8 check.
+    # https://pep8.readthedocs.io/en/latest/intro.html#error-codes
+    # Ignored errors:
+    # - E501: line too long
+    # - E731: do not assign a lambda expression, use a def
+    - pep8 --ignore=E501,E731 .