collections -> collections.abc
collections -> collections.abc

Summary: `/detectron2/evaluation/testing.py:6: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3,and in 3.9 it will stop working` is showing up in the logs

Reviewed By: ppwwyyxx

Differential Revision: D19899433

fbshipit-source-id: 542aeacedd99998672739d2ee264bbb1e1d5fed2

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2