Migrate from google-apputils to absl-py

[gpshead]

Your dependency google-apputils is deprecated. Please migrate to absl-py instead.

[graphicore]

Thanks @gpshead

context: https://github.com/google/google-apputils/pull/15 https://github.com/google/google-apputils/issues/14

[felipesanches]

I believe the code was already migrated, but we still mention the dependency on README.md and also have it commented out on requirements.txt

So, I think there are the last things we need to cleanup before fixing this issue.