heptapod-ci: run test with python3 too

Python3 is the future^W present, it is important to run tests with it too.

Differential Revision: https://phab.mercurial-scm.org/D8013

contrib/heptapod-ci.yml

@@ -1,13 +1,25 b''
 image: octobus/ci-mercurial-core
 
 # The runner made a clone as root.
 # We make a new clone owned by user used to run the step.
 before_script:
     - hg clone . /tmp/mercurial-ci/ --noupdate
     - hg -R /tmp/mercurial-ci/ update `hg log --rev '.' --template '{node}'`
     - cd /tmp/mercurial-ci/
 
-checks-py2:
+variables:
+    PYTHON: python
+
+.runtests_template: &runtests
     script:
         - cd tests/
-        - python run-tests.py --color=always --time test-check-*.*
+        - echo "python used, $PYTHON"
+        - $PYTHON run-tests.py --color=always --time test-check-*.*
+
+checks-py2:
+    <<: *runtests
+
+checks-py3:
+    <<: *runtests
+    variables:
+        PYTHON: python3