Update .travis.yml

Remove python 3.5 and add 3.8

.travis.yml

@@ -5,15 +5,15 @@ install:
 script: tox
 matrix:
         include:
-                - os: "linux"
-                  dist: "xenial"
-                  python: "3.5"
                 - os: "linux"
                   dist: "xenial"
                   python: "3.6"
                 - os: "linux"
                   dist: "xenial"
                   python: "3.7"
+                - os: "linux"
+                  dist: "xenial"
+                  python: "3.8"
                 - os: "windows"
                   language: shell
                   python: "3.7"