ensure python exec can be config. for gitlab CI

.gitlab-ci.yml

@@ -62,7 +62,7 @@ test-centos7:
   - coverage html
   - pip3.6 install sphinx sphinx-argparse sphinxcontrib.bibtex sphinx_rtd_theme
   - cd docs
-  - make html
+  - PYTHON=python3.6 make html
   artifacts:
     paths:
       - ippisite/htmlcov

ippisite/docs/Makefile

@@ -19,5 +19,5 @@ help:
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	@$(SPHINXAPI) -o $(SOURCEDIR) $(SPHINXAPISRC)
-	python ../manage.py graph_models ippidb --dot > source/db.dot
+	$(PYTHON) ../manage.py graph_models ippidb --dot > source/db.dot
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)