Fix typo: collections.abs -> collections.abc

[earwig]

collections.abs does not exist, and surely collections.abc was meant instead. (Introduced in #1387.)

This causes a warning in recent versions of Python 3.

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

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[earwig]

I should have noticed #1443. Whoops!