travis: use newer dists, native py on dist

also remove double-test of py39

.travis.yml

@@ -7,33 +7,29 @@ cache:
 matrix:
     fast_finish: true
     include:
-        - python: "3.5.2"  # Use version 3.5.2 because it has lzma support while 3.5(.0) does not
+        - python: "3.5"
           os: linux
           dist: xenial
           env: TOXENV=py35
         - python: "3.6"
           os: linux
-          dist: xenial
+          dist: bionic
           env: TOXENV=py36
         - python: "3.7"
           os: linux
-          dist: xenial
+          dist: bionic
           env: TOXENV=py37
         - python: "3.8"
           os: linux
-          dist: xenial
+          dist: focal
           env: TOXENV=py38
-        - python: "3.9-dev"
-          os: linux
-          dist: xenial
-          env: TOXENV=py39
         - python: "3.9-dev"
           os: linux
           dist: focal
           env: TOXENV=py39
-        - python: "3.6"
+        - python: "3.8"
           os: linux
-          dist: xenial
+          dist: focal
           env: TOXENV=flake8
         - language: generic
           os: osx