Don't allow checking if None is within the bounds (#1140)

This was never supported, a `ValueError` would be raised if `None` was
passed to `__contains__`. Now we assert to make it clear that it's not
allowed.

In the future we should probably change `Bounds` to always allow `None`
as bounds and make `_T` simply bound to `Comparable`, as supporting
`None` depending on the generic type makes it very hard to provide
correct type hints.

src/frequenz/sdk/timeseries/_base_types.py

@@ -165,7 +165,13 @@ def __ge__(self, other: Any, /) -> bool:
 
 @dataclass(frozen=True)
 class Bounds(Generic[_T]):
-    """Lower and upper bound values."""
+    """Lower and upper bound values.
+
+    Depending on the genertic type `_T`, the lower and upper bounds can be `None`, in
+    which case it means that there is no lower or upper bound, respectively.
+
+    When checking if an item is within the bounds, the item must always be not `None`.
+    """
 
     lower: _T
     """Lower bound."""
@@ -178,11 +184,12 @@ def __contains__(self, item: _T) -> bool:
         Check if the value is within the range of the container.
 
         Args:
-            item: The value to check.
+            item: The value to check. Can't be `None` even if `_T` can be `None`.
 
         Returns:
             bool: True if value is within the range, otherwise False.
         """
+        assert item is not None, "Can't check if `None` is within the bounds."
         if self.lower is None and self.upper is None:
             return True
         if self.lower is None: