[multiprocessing] fixes more incorrect types around context

[vishalkuo]

Another iteration on #4266 which should knock out a couple issues (see diff output). Comments on the fixes:

multiprocessing.JoinableQueue is not a type multiprocessing.Queue is not a type multiprocessing.SimpleQueue is not a type 

These are all actually functions in the context class that return queues from the queues module here.

multiprocessing.Event is inconsistent, runtime does not have argument "lock" multiprocessing.set_executable is inconsistent, stub argument "exe" differs from runtime argument "executable" 

These are also functions in context that differ from runtime: see event and set_executable

Test Plan:

(mypy) (⎈ |development:home)[~/projects/typeshed] (mp) $ git co HEAD~1 (mypy) (⎈ |development:home)[~/projects/typeshed] ((HEAD detached at 5d553c95)) $ python3.8 -m mypy.stubtest --custom-typeshed-dir . multiprocessing --concise > ~/Desktop/pre-change.txt (mypy) (⎈ |development:home)[~/projects/typeshed] ((HEAD detached at 5d553c95)) $ git co mp Previous HEAD position was 5d553c95 apply black and isort (#4287) Switched to branch 'mp' (mypy) (⎈ |development:home)[~/projects/typeshed] (mp) $ python3.8 -m mypy.stubtest --custom-typeshed-dir . multiprocessing --concise > ~/Desktop/post-change.txt (mypy) (⎈ |development:home)[~/projects/typeshed] (mp) $ diff ~/Desktop/pre-change.txt ~/Desktop/post-change.txt 1,3d0 < multiprocessing.Event is inconsistent, runtime does not have argument "lock" < multiprocessing.JoinableQueue is not a type < multiprocessing.Queue is not a type 6d2 < multiprocessing.SimpleQueue is not a type 8,9d3 < multiprocessing.set_executable is inconsistent, stub argument "exe" differs from runtime argument "executable" < multiprocessing.context.BaseContext.Event is inconsistent, runtime does not have argument "lock" 

[hauntsaninja]

Nice!