Merge branch 'py36' into 'master'

Switch to Python 3.6 as runtime python

See merge request !908

Dockerfile

-FROM python:3.5
+FROM python:3.6
 MAINTAINER Thom Wiggers <thom@thomwiggers.nl>
 LABEL description="Contains the Thaliawebsite Django application"
 

tox.ini

 [tox]
-envlist = flake8, {py35,py36,py37}-django20, {py35,py36,py37}-django21
+envlist = flake8, {py36,py37}-django20, {py36,py37}-django21
 skipsdist = True
 skip_missing_interpreters = True
 
@@ -26,6 +26,7 @@ env =
     PIPENV_SUPPRESS_NESTED_WARNING=1
 
 [testenv:flake8]
+basepython=python36
 deps= flake8
 skip_install=True
 commands=