Add mypy environment to tox configuration

Author: mthuurne

tox.ini

@@ -8,11 +8,12 @@ envlist =
     py{310,311,312}-dj{main}
     flake8
     isort
+    mypy
 
 [gh-actions]
 python =
     3.7: py37
-    3.8: py38, flake8, isort
+    3.8: py38, flake8, isort, mypy
     3.9: py39
     3.10: py310
     3.11: py311
@@ -63,3 +64,13 @@ deps = isort
 commands =
     isort model_utils tests setup.py --check-only --diff
 skip_install = True
+
+[testenv:mypy]
+basepython = python3.8
+deps =
+    time-machine==2.8.2
+    -r requirements-mypy.txt
+set_env =
+    SQLITE=1
+commands =
+    mypy model_utils tests