Update multiprocessing.managers.ListProxy and multiprocessing.managers.DictProxy

Feature or enhancement

Standard lists accept clear as a shortcut for del xs[:]

However, multiprocessing.ListProxy omitted support for clear().

from multiprocessing import Manager

with Manager() as manager:
    xs = manager.list()
    xs.clear()

raises the following exception

AttributeError: 'ListProxy' object has no attribute 'clear'

Pitch

If clear is supported in a standard list, it should be supported in a multiprocessing list!

Previous discussion

This issue was not previously discussed.

Linked PRs