Travis CI: Test on Python 3.9 release candidate 2

Python 3.9 is now on its release candidate 2 and is scheduled to be released two weeks so let's start testing to see if we are compatible with our future...

Author: cclauss

.travis.yml

@@ -1,7 +1,7 @@
 os: linux
 dist: focal
 language: python
-python: 3.8
+python: 3.9-dev
 cache: pip
 before_install: pip install --upgrade pip setuptools six
 install: pip install black flake8