[python] Make the collections import future-proof

On Python 3.7 the old code raises a warning:

	DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    	class ArgumentsIterator(collections.Sequence):

On Python 3.8 it wouldn't work anymore.

Commited on behalf of Jakub Stasiak.

Differential Revision: https://reviews.llvm.org/D56341

llvm-svn: 350467