travis: Upgrade to bionic

.travis.yml

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: MIT
 ---
-dist: xenial
+dist: bionic
 language: python
 env:
   global:
@@ -11,7 +11,6 @@ matrix:
     - python: 2.6
       dist: trusty
     - python: 2.7
-    - python: 3.5
     - python: 3.6
     - python: 3.7
     - python: 3.8

tox.ini

@@ -270,7 +270,6 @@ max-line-length = 88
 python =
   2.6: py26,coveralls,custom
   2.7: py27,coveralls,flake8,pylint,custom
-  3.5: custom
   3.6: py36,coveralls,black,yamllint,custom
   3.7: py37,coveralls,custom
   3.8: py38,coveralls,custom