add annotation for multiprocessing.{Value,Array} with special c-types

[prauscher]

This PR adds support for multiprocessing.Value with standard c-types like c_float. Before this PR, Code like

from multiprocessing import Value
from ctypes import c_float
a = Value(c_float, 0.0)
a.value = 1.2 

lead to incompatible types in assignment.

Also this PR resolves a Problem addressed in #4266 where

from multiprocessing import Array
from ctypes import c_char
a = Array(c_char, 40)
print(a.value)

lead to SynchronizedArray[c_char] has no attribute value, altrough documentation states that such arrays have a raw and a value field.

Happy for your comments, lets see what I missed :)

[srittau]

You removed the type "wrapper" here, while Array still has it, was this intentional?

Also, could you add some test cases to test_cases/stdlib? I think the multiprocessing stubs are sufficiently complex to warrant some.

[prauscher]

this was intentional and a misstake, which is why i re-added them now. I agree with test-cases, let's see what I can come up with.

[prauscher]

What I could not get working:

from multiprocessing import Array
from ctypes import c_float
a = Array(c_float, 2)
a[:] = [1.2, 4.1]  # Incompatible types: expect c_float, found float

Sadly, adding a new overload for this like it is done with Value breaks, so this requires a bit more thinking

[srittau]

I suggest to add the examples from your initial comment to the tests, see below.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉