Add python 3.9 testing (#351)

* Add python 3.9 testing

* Add 3.9 proper

According to this, it's available now: https://travis-ci.community/t/python-3-9-0-build/10091/18

.travis.yml

@@ -6,6 +6,7 @@ python:
   - 3.6
   - 3.7
   - 3.8
+  - 3.9
   - pypy3
 
 install:
@@ -24,4 +25,4 @@ jobs:
   include:
     - python: 3.8
       install: pip install -U black
-      script: black --check .
+      script: black --check .

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36, py37, py38, pypy3
+envlist = py36, py37, py38, py39, pypy3
 
 [testenv]
 deps=-r{toxinidir}/requirements/dev.pip
@@ -8,7 +8,7 @@ allowlist_externals=
     mypy
 sitepackages=False
 
-[testenv:py{36,37,38}]
+[testenv:py{36,37,38,39}]
 commands=
     /bin/sh -c "{env:PYPISERVER_SETUP_CMD:true}"
     # individual mypy files for now, until we get the rest