Use Python 3.9 instead of 3.6 for Automerge checking

Updates `travis.yml` file to use Pyhton3.9 instead of 3.6 in the "Automerge check" job. Using Python 3.6 causes dependency issues with BSL's requirement for `setuptools` version and the ones available in PyPI for Python 3.6.

.travis.yml

@@ -55,6 +55,7 @@ jobs:
       python: '3.9'
       env: PRIVATE_ACCESS=1
     - stage: "Automerge check"
+      python: '3.9'
       install: python -m pip install -e ".[test]"
       if: type = pull_request
       script: