Tweak Tox workflow

- No need to specify the environment
- MyPy has its own config file
- MyPy runs for Python 3.9

Author: blyxxyz

CONTRIBUTING.md

@@ -5,7 +5,7 @@
 [`tox`](https://tox.readthedocs.io/en/latest/) is used to run the tests and linters. After installing it, run:
 
 ```
-tox -e py3
+tox
 ```
 
 This will install and run all the tooling.

mypy.ini

@@ -0,0 +1,3 @@
+# -*- conf -*-
+[mypy]
+strict = True

tox.ini

@@ -1,16 +1,17 @@
 [tox]
-envlist = py34, py35, py36, py37, py38, pypy3
+envlist = py3
 toxworkdir = {env:TOX_WORK_DIR:.tox}
 
 [testenv]
 deps =
     pytest
     flake8
-    py{3,35,36,37,38}: mypy
+    py{3,35,36,37,38,39}: mypy
     hypothesis
 commands =
     flake8 nnf test_nnf.py
-    py{3,35,36,37,38}: python -m mypy --strict nnf
+    py{3,35,36,37,38,39}: mypy nnf
+    py{3,35,36,37,38,39}: mypy --python-version=3.4 nnf
     pytest
-    py{3,36,37,38,py3}: python -m doctest nnf/__init__.py -o ELLIPSIS
+    py{3,36,37,38,39,py3}: python -m doctest nnf/__init__.py -o ELLIPSIS
     python examples/socialchoice.py