RET504 false positive via decorator

[charliermarsh]

Here's an interesting case where variable is assigned to, referenced, and then returned:
https://github.com/KyleKing/pytest_cache_assert/blob/47d0b538528a9c9cdbc182d2b276462a2aa0fd4d/tests/test_flask.py#L20

def _create_app() -> Flask:
    app = Flask(__name__)

    @app.route('/hello')
    def hello() -> str:
        """Hello endpoint."""
        return 'Hello, World!'

    return app  # noqa: RET504

Seems like something that shouldn't be flagged.

Perhaps instead of an assignment count, heuristic could be reference count. In other words, excluding return statement, flag if the only reference was assignment.

Originally posted by @gdub in #2950 (comment)