Set up MyPy for CI

pyproject.toml

@@ -50,3 +50,35 @@ include-package-data = true
 test = [
     "pytest",
 ]
+mypy = [
+    "mypy",
+    {include-group = "test"},
+]
+
+
+# Mypy
+# ====
+
+[tool.mypy]
+files = "."
+exclude = [
+  "build/",
+  "docs/",
+  "venv",
+]
+
+pretty = true
+show_error_codes = true
+
+enable_error_code = [  # pragma: alphabetize
+  "ignore-without-code",
+  "possibly-undefined",
+  "truthy-bool",
+  "truthy-iterable",
+]
+strict = false
+ignore_missing_imports = true
+no_implicit_optional = true
+strict_equality = true
+warn_unreachable = true
+warn_no_return = true

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py3{9,10,11,12,13,14}
+envlist = py3{9,10,11,12,13,14},mypy
 
 [testenv]
 # Install wheels instead of source distributions for faster execution.
@@ -9,3 +9,9 @@ wheel_build_env = .pkg
 
 dependency_groups = test
 commands = python -m pytest {posargs:tests}
+
+[testenv:mypy]
+dependency_groups = mypy
+setenv =
+    TERM=xterm-color
+commands = mypy {posargs:.}