Update Django and Python build versions

* Add Django 3.1 builds
* Django main is now 3.2dev; add test settings
* Rough in changelog deprecation notice for Django 1.11
  and Python 2.7
* Travis-CI is apparently supporting pypy3 alias now
* Travis-CI has apparently added Python 3.8 release now

tox.ini

@@ -3,17 +3,18 @@ envlist =
     # Factors: django-python-extras
     # Test these environments first, to catch most errors early...
     lint
-    django22-py37-all
+    django30-py37-all
     django111-py27-all
     docs
     # ... then test all the other supported combinations:
-    django22-py{35,36,py3}-all
+    django30-py{36,38,py3}-all
+    django22-py{35,36,37,py3}-all
     django21-py{35,36,37,py3}-all
     django20-py{35,36,py3}-all
     django111-py{34,35,36,py}-all
     # ... then prereleases (if available):
-    django30-py{36,37,38,py3}-all
-    djangoMaster-py{36,37,38}-all
+    django31-py{36,37,38,py3}-all
+    djangoDev-py{36,37,38}-all
     # ... then partial installation (limit extras):
     django22-py37-{none,amazon_ses,sparkpost}
     # ... then older versions of some dependencies:
@@ -27,8 +28,8 @@ deps =
     django21: django~=2.1.0
     django22: django~=2.2.0
     django30: django~=3.0.0
-    django31: django>=3.1a1
-    djangoMaster: https://github.com/django/django/tarball/master
+    django31: django>=3.1rc1
+    djangoDev: https://github.com/django/django/tarball/master
     old_urllib3: urllib3<1.25
     # testing dependencies (duplicates setup.py tests_require, less optional extras):
     mock
@@ -42,7 +43,7 @@ setenv =
     sparkpost: ANYMAIL_ONLY_TEST=sparkpost
 ignore_outcome =
     # CI that wants to handle errors itself can set TOX_FORCE_IGNORE_OUTCOME=false
-    djangoMaster: {env:TOX_FORCE_IGNORE_OUTCOME:true}
+    djangoDev: {env:TOX_FORCE_IGNORE_OUTCOME:true}
 args_are_paths = false
 commands_pre =
     python -VV