[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!