add 3.9-dev instead of 3.9

Author: seyong92

.travis.yml

@@ -3,7 +3,7 @@ python:
   - "3.6"
   - "3.7"
   - "3.8"
-  - "3.9"
+  - "3.9-dev"
 env:
   - NUMPY_VERSION=1.16.6
   - NUMPY_VERSION=1.17.5