bindings: python: cast return value of LineRequest.get_values

bindings: python: cast return value of LineRequest.get_values

The `values` argument of `_ext.Request.get_values` uses a preallocated
`list[None]` as a buffer that is populated with `Value`s by the external
module that are then returned from the function.

Use `cast` to inform the type checker it's a `list[Value]` despite how
it's allocated.

Also, as `lines` is typed as an `Iterable`, there is no guarantee it has
a `__len__` method. Instead, use the size of the `offsets` array to
allocate the buffer.

Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
Link: https://lore.kernel.org/r/20241114145116.2123714-14-vfazio@xes-inc.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>