Add support for Python 3.9

This also updates the tox minversion to a very recent version
in hopes of triggering tox provisioning, which seems to avoid
tox-dev/tox#1484

Author: nayanzin33sergey

.github/workflows/validation.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2

setup.py

@@ -39,7 +39,8 @@ def read(fname):
         'filelock>=3.0',
         'pytest>=3.5',
         'mypy>=0.500; python_version<"3.8"',
-        'mypy>=0.700; python_version>="3.8"',
+        'mypy>=0.700; python_version>="3.8" and python_version<"3.9"',
+        'mypy>=0.780; python_version>="3.9"',
     ],
     classifiers=[
         'Development Status :: 4 - Beta',
@@ -52,6 +53,7 @@ def read(fname):
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: Implementation :: CPython',
         'Operating System :: OS Independent',
         'License :: OSI Approved :: MIT License',

tox.ini

@@ -1,12 +1,13 @@
 # For more information about tox, see https://tox.readthedocs.io/en/latest/
 [tox]
-min_version = 3.7.0
+minversion = 3.20
 isolated_build = true
 envlist =
     py35-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.50, 0.5x, 0.60, 0.6x, 0.70, 0.7x}
     py36-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.50, 0.5x, 0.60, 0.6x, 0.70, 0.7x}
     py37-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.50, 0.5x, 0.60, 0.6x, 0.70, 0.7x}
     py38-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.71, 0.7x}
+    py39-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.78, 0.7x}
     publish
     static
 
@@ -16,6 +17,7 @@ python =
     3.6: py36-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.50, 0.5x, 0.60, 0.6x, 0.70, 0.7x}
     3.7: py37-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.50, 0.5x, 0.60, 0.6x, 0.70, 0.7x}
     3.8: py38-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.71, 0.7x}, publish, static
+    3.9: py39-pytest{3.5, 3.x, 4.0, 4.x, 5.0, 5.x, 6.0, 6.x}-mypy{0.78, 0.7x}
 
 [testenv]
 deps =