MyPy: enable truthy-bool

This catches mistakes in using a value as truthy if it cannot be falsey.

pyproject.toml

@@ -90,7 +90,7 @@ exclude = [
 [tool.mypy]
 disallow_untyped_defs = false
 disallow_incomplete_defs = false
-enable_error_code = ["ignore-without-code"]
+enable_error_code = ["ignore-without-code", "truthy-bool"]
 follow_imports = "normal"
 ignore_missing_imports = true
 pretty = true