[Train] Replace abc.ABCMeta with abc.ABC in callbacks

[bveeramani]

Why are these changes needed?

Inheriting from abc.ABC is more readable than setting the meta class to abc.ABCMeta.

Relevant snippet from the Python 3.4 release notes:

New class ABC has ABCMeta as its meta class. Using ABC as a base class has essentially the same effect as specifying metaclass=abc.ABCMeta, but is simpler to type and easier to read. (Contributed by Bruno Dupuis in bpo-16049.)

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[bveeramani]

The lint status check is failing because there's apparently a formatting issue with logging.py. I ran ./scripts/format.sh, but it didn't reformat logging.py. I also manually checked the yapf version and ran yapf --style .style.yapf ./python/ray/train/callbacks/logging.py, but it also didn't make any changes. This might be due to some non-deterministic yapf nonsense?

In any case, will debug when I get back from OOO.

[richardliaw]

@bveeramani fixed it for you (the lint ran fine on my computer and generated the right diff)