add py=3.9 to travisCI builds

.travis.yml

@@ -3,6 +3,7 @@ python:
   # We don't actually use the Travis Python, but this keeps it organized.
   - "3.7"
   - "3.8"
+  - "3.9"
 install:
   - sudo apt update
   # We do this conditionally because it saves us some downloading if the