Drop Python 2.6 support and add 3.7

PIP requires Python 2.7, so it is not possible to test using anything prior to that.

Python 3.7 has been out for a while, so we can test against it.

.gitlab-ci.yml

@@ -79,6 +79,19 @@ python3.5:
 
 python3.6:
   image: gitlab-registry.cern.ch/lhcb-docker/python-deployment:python-3.6
+  script:
+    - python --version
+    - python setup.py nosetests --cover-package LbPlatformUtils
+    - mkdir -p cover_report && mv -f build/lib/cover cover_report/$CI_JOB_NAME
+    - python setup.py install && lb-describe-platform --flags
+  artifacts:
+    paths:
+      - cover_report
+    when: always
+    expire_in: 1 week
+
+python3.7:
+  image: gitlab-registry.cern.ch/lhcb-docker/python-deployment:python-3.7
   script:
     - python --version
     - python setup.py nosetests --cover-package LbPlatformUtils