pyproject.toml: Set disallow_incomplete_defs = true

With the parent of this commit, we pass this check.

pyproject.toml

@@ -46,5 +46,6 @@ line-length=100
 
 [tool.mypy]
 check_untyped_defs = true
+disallow_incomplete_defs = true
 disallow_untyped_calls = true
 disallow_untyped_decorators = true