Improve python.syntax with new keywords

[baryluk]

nonlocal:

Added in Python 3.0 in 2006:

​https://docs.python.org/3.0/whatsnew/3.0.html
​https://www.python.org/dev/peps/pep-3104/
​https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement

async, await:

Added in Python 3.5 in 2015:

​https://docs.python.org/3.5/whatsnew/3.5.html#what-s-new-in-python-3-5
​https://www.python.org/downloads/release/python-350/
​https://www.python.org/dev/peps/pep-0492/
​https://docs.python.org/3/reference/compound_stmts.html#async-def
​https://docs.python.org/3/reference/compound_stmts.html#the-async-for-statement
​https://docs.python.org/3/reference/compound_stmts.html#the-async-with-statement
​https://docs.python.org/3/reference/expressions.html#await-expression
​https://docs.python.org/3/glossary.html#term-coroutine
​https://docs.python.org/3/library/asyncio-task.html

[zaytsev]

How about the walrus operator?

[baryluk]

Replying to zaytsev:

How about the walrus operator?

Maybe in another patch another time. :) := shows okish, but not perfect, so yeah, it should be fixed.

I will probably also make patch to show the f-string literals nicer, i.e. by using different highlighting of the content of {} in them. But that requires a bit of tricks to get rite in syntax file.

I just wanted to keep the patch small and easy to review / merge.

[andrew_b]

Thanks!

Applied as [b01fea54a7eaf58a179ba7c47687e0da1f2c301c].