Use Python 3.8 instead of 3.5

.travis.yml

@@ -3,7 +3,7 @@ language: python
 dist: xenial  # required for Python >= 3.7
 
 python:
-  - "3.5"
+  - "3.8"
   - "3.7"
 # Multiple "Test" jobs seems to be required to run tests when
 # conditional stages are added.  Run the CI with only one `python`