Added a minor comment on mypy to dev guide

Author: semohr

CONTRIBUTE.md

@@ -52,7 +52,7 @@ Please make sure the tests pass and the code is linted before submitting a pull
 pip install pre-commit
 pre-commit install
 ```
-
+^
 This should now automatically reject commits that do not pass the linting and formatting checks.
 
 You may run our tests locally with:
@@ -62,6 +62,12 @@ pip install .[dev,test]
 pytest
 ```
 
+Further if you want to make sure that the types are correct, you can run:
+```bash
+cd backend
+mypy beets_flask
+```
+
 ## Additional Resources
 
 See also [Resources.md](./RESOURCES.md) for some background information and design choices.

backend/pyproject.toml

@@ -84,7 +84,7 @@ pythonpath = ["."]
 omit = ["*/tests/*"]
 
 [tool.mypy]
-check_untyped_defs = false
+check_untyped_defs = true
 
 [[tool.mypy.overrides]]
 # Suppresses error messages about imports that cannot be resolved.