Address mypy complaint about implicit optional argument

Make it explicit the argument is optional.

Author: pavoljuhas

tunits/core/__init__.pyi

@@ -337,7 +337,7 @@ class ValueArray(Generic[ValueType2], WithUnit):
     def __init__(self, data: Any, unit: Any = None) -> None: ...
     def allclose(self, other: ValueArray | Value, *args: Any, **kwargs: float) -> bool: ...
     def unique(self: ArrayType) -> ArrayType: ...
-    def __array__(self, dtype: DTypeLike = None) -> NDArray[Any]: ...
+    def __array__(self, dtype: DTypeLike | None = None) -> NDArray[Any]: ...
     def __array_wrap__(self, out_arr: NDArray[Any], context: Any = None) -> NDArray[Any]: ...
     def __copy__(self: ArrayType) -> ArrayType: ...
     def __deepcopy__(self: ArrayType) -> ArrayType: ...