As we are just out of a while loop whose stopping condition is that
`sprite_lists` is empty, calling `clear()` on it is redundant.

There was already a method `register_sprite_list` to handle all
additions to `sprite_lists`. We add a corresponding method
`_unregister_sprite_list` to handle removals.

We make the typings of these methods stricter, so that we can
enforce the correct typing invariant on `sprite_lists`. We also
make that invariant clearer in a comment.

`sprite_lists` is unfortunately unsafely visible to everyone.
So a user of the class could still violate the invariants. At least
now the *intended* usage is safe.

This is similar to the fix done to `check_for_collision_with_list`
done in c387717.

Adding type parameters to some `SpriteList`s. One allows to get rid
of a cast.

This is done by analogy to `collections.abc.Sequence`, which is a
covariant supertype of `list`.

Before this commit, many parts of the codebase used `SpriteList`s
without type arguments (defaulting to `Unknown`). That was the only
way to allow reasonable usages of the given methods and attributes.
However, doing so results in weaker typing.

Using `SpriteSequence`, we can add correct type arguments to almost
all of the references that were using `SpriteList`s before.

The only missing pieces are `Scene` and `TileMap`. Unfortunately,
their APIs are fundamentally unsound wrt. the type arguments of
their `SpriteList`s. We cannot make it sound without breaking their
APIs, so we do not change them.

As a bonus, we can now create lists of `SpriteList`s with varying
type arguments, and generically call `draw` or `update` on them.
Previously, the only common supertype of `SpriteList[A]` and
`SpriteList[B]` was `object`, which meant it was not possible to
call those methods on them.

In a sense, that ability mostly subsumes the convenience provided
by `Scene`. A `list[SpriteSequence[BasicSprite]]` is almost as
convenient, while being type-safe.