Try workaround recommended here https://github.com/travis-ci/travis-ci/issues/9815

.travis.yml

@@ -5,7 +5,12 @@ cache: pip
 python:
   - '2.7'
   - '3.6'
-  - '3.7'
+
+matrix:
+  include:
+  - python: 3.7
+    dist: xenial
+    sudo: true
 
 install:
   - pip install -U pip setuptools
@@ -14,5 +19,3 @@ install:
 script:
   - python setup.py test
   - flake8 disco/
-
-dist: xenial