start using koji.ClientSession.multicall as a context manager

[obriencj]

since koji >= 1.18 multicall has been available to use as a context manager. Let's start taking advantage of that feature finally.

[obriencj]

we should also take care to finally document that interface in our typing stubs

[obriencj]

this makes it very difficult to work with mypy for type checking, as the managed interface context manager returns result wrappers rather than the normal values from the calls. I'd need to write some sort of specialty type that decorated the entire signature of the client session.

[obriencj]

solved the mypy issue with a custom plugin. nothing needs converting to context currently. closing.