Travis CI: add Python 3.9 (#85)

vstinner · web-flow · commit fea75415f8da · 2020-11-26T18:57:20.000+01:00
Also replace Python 3.7 with 3.6
diff --git a/.travis.yml b/.travis.yml
@@ -2,7 +2,9 @@ language: python
 dist: xenial
 cache: pip
 python:
-    - 3.7
+    # minimum required Python version
+    - 3.6
+    - 3.9
     - nightly
     - pypy3
 install:
