[docs] [issue28635] Update What's New for 3.6

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Thank you Yury and Elvis for working on this!

I have few more suggestions for What's New:

* collections.abc.Reversible (http://bugs.python.org/issue25987).

* various ABCs in collections.abc now have means for explicit "anti-registration" by setting a corresponding attribute to None (http://bugs.python.org/issue25958, https://docs.python.org/3.6/reference/datamodel.html#special-method-names).

* implementation improvements (such as caching of generic types) in typing module give up to 30x seep-up and reduce memory footprint.

* typing.py now supports generic type aliases (https://github.com/python/typing/pull/195 and https://github.com/python/typing/pull/308, see mypy docs for usage examples).

* typing.py supports PEP 526 syntax for typed NamedTuple (https://github.com/python/typing/pull/282).

(I see that there are already several items about typing, so please feel free to keep only the most important changes.)

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nosy: +levkivskyi

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