test on Python 3.9

Author: mr-c

.travis.yml

@@ -1,8 +1,12 @@
+branches:
+  only:
+    - main
 language: python
 python:
 - '3.6'
 - '3.7'
 - '3.8'
+- '3.9'
 before_install:
 - pip install .[toil]
 - pip install -r dev-requirements.txt