Add types-pyyaml as test dependency

[InvincibleRMC]

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Currently https://github.com/ros2/launch/pull/781 is failing because it cannot find types-pyyaml this dependency was added to rosdep here https://github.com/ros/rosdistro/pull/40651. I could be mistaken but, I believe the proper course of action is maunally adding the dep here.

[InvincibleRMC]

@clalancette Any update on this? Anything else you want from me?

[MichaelOrlov]

@cottsay Friendly ping here to reiterate on review and merge it if it is Okay.

[ahcorde]

friendly ping @cottsay