Run test on Python 3.7

Run these on a synthetic environment, since no distribution seems to
currently ship this version.

Author: Hugo Osvaldo Barrera

.gitlab-ci.yml

@@ -0,0 +1,6 @@
+python37:
+  image: python:3.7
+  before_script:
+    - make -e install-dev
+  script:
+    - make -e ci-test