Drop Py2.6 CI

.gitlab-ci.yml

-Python 2.6:
-  script:
-  - py_version=2.6
-  - export EXTRA_INSTALL="numpy"
-  - curl -L -O -k https://gitlab.tiker.net/inducer/ci-support/raw/master/build-and-test-py-project.sh
-  - ". ./build-and-test-py-project.sh"
-  tags:
-  - python2.6
-  except:
-  - tags
-  artifacts:
-    reports:
-      junit: test/pytest.xml
-
 Python 2.7:
   script:
   - py_version=2.7