[docs] [issue32996] Improve What's New in 3.7

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

The most breaking change in 3.7 (according to the statistics of GitHub issues) is making async a keyword. But this change is even not mentioned in What's New.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32996>
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