Support building under Python 3.8

[na-Itms]

typed_ast supports 3.8 from 1.4.0 onwards, while
mypy supports typed_ast>=1.4.0 since 0.710.

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Checklist

• I read the commit guidelines and I've followed
  them.
• I ran coala over my code locally. (All commits have to pass
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[na-Itms]

(this supersedes #2968, by the way)

[utkarsh2102]

Me likey, though worth rebasing but that's really trivial. As long as everything "works" and nothing breaks, it's good to go! 🎉