Add type annotations to `client.dynamic.Dispatch`

[junkmd]

Remaining tasks for #443 and #444.

https://github.com/enthought/comtypes/blob/11410ce563634b9c96b6ba9b30ac95e6a3af1f97/comtypes/client/dynamic.py#L22-L34

Before PR, please format the code by black (the maximum number of characters per line is 88, which is the default).