Set up mypy

setup.cfg

@@ -55,6 +55,7 @@ tox =
 testing =
     black; platform_python_implementation=='CPython'
     flake8 >=3, <4
+    mypy
     pytest >=4, <6
     pytest-cov >=2, <3
     pytest-mock >=2, <3
@@ -74,14 +75,15 @@ skip_missing_interpreters = true
 envlist =
     black
     flake8
+    mypy
     {py36,py37,py38,py39,pypy2,pypy3}-tox{312,315,latest}
 
 [gh-actions]
 python =
     3.6: py36
     3.7: py37
     3.8: py38, black, flake8
-    3.9: py39
+    3.9: py39, mypy
     pypy-2: pypy2
     pypy-3: pypy3
 
@@ -103,5 +105,13 @@ description = run flake8 under {basepython}
 commands = flake8 src/ tests/ setup.py
 extras = testing
 
+[testenv:mypy]
+description = run mypy under {basepython}
+commands = flake8 src/ tests/ setup.py
+extras = testing
+
 [flake8]
 max-line-length = 88
+
+[mypy]
+ignore_missing_imports = True