heptapod-ci: indicate which version of black is used for the run

This will help understand possible mismatch

Differential Revision: https://phab.mercurial-scm.org/D9887

contrib/heptapod-ci.yml

@@ -20,6 +20,7 b' variables:'
       - cargo build
       - cd /tmp/mercurial-ci/
       - ls -1 tests/test-check-*.* > /tmp/check-tests.txt
+      - black --version
     script:
         - echo "python used, $PYTHON"
         - echo "$RUNTEST_ARGS"