Test on Travis with Python 3.7

.travis.yml

+dist: xenial
 language: python
 python:
-    - 3.6
+    - 3.7
 sudo: required
 services:
     - docker
 install:
+    - pip install --upgrade pip
     - make dev-env
 script:
     - set -e
-    - make test/docs docs
+    - make docs
     - make build-test-all DARGS="--build-arg TEST_ONLY_BUILD=1"

Makefile

@@ -60,9 +60,6 @@ dev-env: ## install libraries required to build docs and run tests
 docs: ## build HTML documentation
 	make -C docs html
 
-test/docs: ## check links in Sphinx documentation
-	make -C docs
-
 test/%: ## run tests against a stack
 	@TEST_IMAGE="$(OWNER)/$(notdir $@)" pytest test