Merge branch 'admin-211-use-python3.7' into 'master'

Use python3.7 as 3.6 image will stop being built nightly and will therefore go stale.

See merge request !27

Dockerfile

 # The docker image built from this Dockerfile is intended only to run the test
 # suite and to build the documentation. Installation of this project should be
 # performed via pip.
-FROM registry.gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/python:3.6-alpine
+FROM registry.gitlab.developers.cam.ac.uk/uis/devops/infra/dockerimages/python:3.7-alpine
 ADD ./ /usr/src/app/
 WORKDIR /usr/src/app
 RUN pip install tox && pip install -e .

tox.ini

 [tox]
-envlist=flake8,doc,py36
+envlist=flake8,doc,py37
 
 # The "_vars" section is ignored by tox but we place some useful shared
 # variables in it to avoid needless repetition.
@@ -29,7 +29,7 @@ commands=
 
 # Build documentation
 [testenv:doc]
-basepython=python3.6
+basepython=python3.7
 deps=
     -rdoc/requirements.txt
 commands=
@@ -38,7 +38,7 @@ commands=
 
 # Check for PEP8 violations
 [testenv:flake8]
-basepython=python3.6
+basepython=python3.7
 deps=
 #   We specify a specific version of flake8 to avoid introducing "false"
 #   regressions when new checks are introduced. The version of flake8 used may