Merge branch 'python37' into 'master'

Use Python 3.7 for Docker builds

See merge request !1148

Dockerfile

-FROM python:3.6
-MAINTAINER Thom Wiggers <thom@thomwiggers.nl>
+FROM python:3.7
+MAINTAINER Thalia Technicie <www@thalia.nu>
 LABEL description="Contains the Thaliawebsite Django application"
 
 # Arguments