Upgrade tox/travis config.

.travis.yml

@@ -1,28 +1,15 @@
-sudo: false
-dist: xenial
+dist: bionic
 language: python
-install:
-    - pip install tox
-
-script:
-    - tox -e $TOXENV
-
+cache: pip
+install: pip install tox tox-travis
+script: tox -v
+python:
+- 3.5
+- 3.6
+- 3.7
+- 3.8
 matrix:
   fast_finish: true
   include:
-    # Python version is just for the look on travis.
-    - python: 3.5
-      env: TOXENV=py35
-
-    - python: 3.6
-      env: TOXENV=py36
-
-    - python: 3.7
-      env: TOXENV=py37
-
-    - python: 3.8
-      env: TOXENV=py38
-
-    # Linters
-    - python: 3.6
-      env: TOXENV=flake8
+  - python: 3.8
+    env: TOXENV=flake8

Changelog.md

@@ -4,6 +4,7 @@
 
 - Added Ukraine calendar, by @apelloni.
 - Drop Support for Python 2 - EOL January 1st 2020 (#442).
+- Small cleanup in the ``.travis.yml`` file, thx to @Natim.
 
 ### ISO Registry API Change