fix(pyargus): address typing issues

This addresses some of the issues with inheritance (internal to the rust
module) for signals, and generally making mypy and flake8 happy.

pyargus/tests/test_signals.py

@@ -66,5 +66,5 @@ def test_correctly_create_signals(data: Tuple[List[Tuple[float, AllowedDtype]],
         assert a < len(samples)
         assert b < len(samples)
     else:
-        assert signal.is_empty()  # type: ignore[attr-defined]
-        assert signal.at(0) is None  # type: ignore[attr-defined]
+        assert signal.is_empty()
+        assert signal.at(0) is None