travis: Use Python 3.9-dev

Author: methane

Diff for: .travis.yml

@@ -29,7 +29,7 @@ matrix:
       python: "3.5"
     - env:
         - DB=mysql:5.6
-      python: "3.6"
+      python: "3.9-dev"
     - env:
         - DB=mysql:5.7
       python: "3.7"