Experiment w/ dataclasses (including Py36) (#3423)
* [ci] Also run test_examples in py37
(will revert at the end of the experiment)
* InputExample: use immutable dataclass
* [deps] Install dataclasses for Py<3.7
* [skip ci] Revert "[ci] Also run test_examples in py37"
This reverts commit d29afd9959786b77759b0b8fa4e6b4335b952015.
