Test on Python 3.9-dev (#509)

* Test on Python 3.9-dev

* Upgrade mypy to fix 'syntax error in type comment' with Python 3.9

Author: hugovk

.github/workflows/test.yml

@@ -8,12 +8,12 @@ jobs:
     strategy:
       matrix:
         os: [macos-latest, ubuntu-latest, windows-latest]
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9-dev]
 
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v1
+      uses: actions/setup-python@v2
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install tox

requirements/tests.txt

@@ -1,3 +1,3 @@
 pytest==3.0.2
 pytest-pep8==1.0.6
-mypy==0.730
+mypy==0.782

tox.ini

@@ -4,7 +4,7 @@
 # install tox" and then run "tox" from this directory.
 
 [tox]
-envlist = {py35,py36,py37,py38}-{tests,install},docs,install,py36-docs
+envlist = {py35,py36,py37,py38,py39}-{tests,install},docs,install,py36-docs
 
 [testenv]
 download = true