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.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 00f5619..7d150e2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -33,6 +33,7 @@
 
   - name: "check code style with flake8"
     python: 3.7
+    dist: xenial
     env:
     - TOXENV=lint