Standardise on Python 3.6.

I'm using ElementaryOS which has Python 3.6 as the system Python3.
It's a supported Python version, the system packages will be used and
everything that runs on that version will run on later versions.

.travis.yml

 # vim:ff=unix ts=2 sw=2 ai expandtab
 ---
 language: python
-python: "3.8"
+python: "3.6"
 dist: bionic
 cache:
   pip: true

Documents/Endless/Pipfile

@@ -27,7 +27,7 @@ google-auth-httplib2 = "*"
 google-auth-oauthlib = "*"
 
 [requires]
-python_version = "3.7"
+python_version = "3.6"
 
 [pipenv]
 allow_prereleases = true