test with stable Python 3.7

instead of out-of-date dev version

Author: minrk

.travis.yml

@@ -63,7 +63,7 @@ matrix:
     include:
         - python: 3.5
           env: GROUP=python
-        - python: "3.7"
+        - python: 3.7
           env: GROUP=python
           dist: xenial  # required for Python >= 3.7 (travis-ci/travis-ci#9069)
         - python: 3.6