Drop python v2.7 and v3.3 from tests, add v3.7–3.9

Author: Evan D

.travis.yml

@@ -1,10 +1,11 @@
 language: python
 python:
-  - "2.7"
-  - "3.3"
   - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
+  - "3.9"
 install:
   - "pip install ."
 # command to run tests