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pep_519_whatsnew.diff looks pretty good to me, thanks! It would be nice to add a simple example to demonstrate the feature (like we did for PEP 498: https://docs.python.org/3.6/whatsnew/3.6.html#pep-498-formatted-string-literals)
>>> import pathlib
>>> with open(pathlib.Path('README')) as f:
... readme = f.read()
...