feat(deps): Add Django 3.1 support (#109)

Pretty basic, looks like no code changes needed.

https://docs.djangoproject.com/en/3.1/releases/3.1/

Author: crccheck

Diff for: .travis.yml

@@ -54,6 +54,15 @@ jobs:
     - stage: test
       python: 3.8
       env: TOX_ENV=django30-py38
+    - stage: test
+      python: 3.6
+      env: TOX_ENV=django31-py36
+    - stage: test
+      python: 3.7
+      env: TOX_ENV=django31-py37
+    - stage: test
+      python: 3.8
+      env: TOX_ENV=django31-py38
     - stage: lint
       install: pip install black
       script: black --check .

Diff for: Makefile

@@ -53,18 +53,19 @@ resetdb: ## Delete and then recreate the dev sqlite database
 	python $(MANAGE) loaddata sample_data
 
 docker/build: ## Build a full set of Docker images
-docker/build: docker/build/3.0 docker/build/2.2.6 docker/build/2.1.13 docker/build/2.0.13 docker/build/1.11.25 docker/build/1.10.8 docker/build/1.9.13 docker/build/1.8.18
+docker/build: docker/build/3.1 docker/build/3.0 docker/build/2.2.6 docker/build/2.1.13 docker/build/2.0.13 docker/build/1.11.25 docker/build/1.10.8 docker/build/1.9.13 docker/build/1.8.18
 
 docker/build/%:
 	docker build --build-arg DJANGO_VERSION=$* \
 	  -t $(IMAGE):$$(echo "$*" | cut -f 1-2 -d.) .
 
-run: run/3.0
+run: run/3.1
 
 run/%:
 	docker run --rm -p 8000:8000 -it $(IMAGE):$*
 
 docker/publish: ## Publish Docker images to the hub
+	docker push $(IMAGE):3.1
 	docker push $(IMAGE):3.0
 	docker push $(IMAGE):2.2
 	docker push $(IMAGE):2.1
@@ -78,7 +79,7 @@ test/%:
 	docker run --rm -p 8000:8000 -t $(IMAGE):$* make test
 
 bash:
-	docker run --rm -it $(IMAGE):3.0 /bin/sh
+	docker run --rm -it $(IMAGE):3.1 /bin/sh
 
 .PHONY: version
 version:

Diff for: tox.ini

@@ -10,6 +10,7 @@ envlist =
     django21-{py35,py36,py37},
     django22-{py36,py37},
     django30-{py36,py37,py38},
+    django31-{py36,py37,py38},
     # run one of the tests again but with coverage
     coveralls-django21-py37,
 skipsdist = True
@@ -29,6 +30,7 @@ deps =
     django21: Django<2.2
     django22: Django<2.3
     django30: Django<3.1
+    django31: Django<3.2
 
 [testenv:coveralls-django21-py37]
 commands =