bpo-28287: Refactor subprocess.Popen to let a subclass handle IO asynchronously

[rhender007]

https://bugs.python.org/issue28287

[the-knights-who-say-ni]

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[gpshead]

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[brettcannon]

Thanks for the PR, but closing as the CLA has not been signed within the last month. If you do decide to sign the CLA we can re-open this PR.