class Composite should explicitly declare current_child type

[jbcpollak]

class Composite should be declared as:

class Composite(behaviour.Behaviour):
    current_child: Optional[Behaviour]

otherwise current_child is typed as None according to the Composite.init method.

[jbcpollak]

update: I discovered this happens with the mypy rule check_untyped_defs = True enabled in a project. I'm going to look more into that rule and if it can be tweaked on a per-dependency basis.

[jbcpollak]

looking into the check_untyped_defs option, I definitely think its ideal to enable, and it does warn about this problem, as well as others. I can make a PR to clean up the errors it causes.

https://mypy.readthedocs.io/en/stable/config_file.html#confval-check_untyped_defs

[stonier]

Thanks for the heads up!

Fixed in #380.