[3.6] Remove unconditional error on async/await name usage

[ddfisher]

Fixes python/mypy#2959.

[the-knights-who-say-ni]

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[gvanrossum]

LGTM. Love the irony of the bot's message, I think you can still merge (but maybe contact Gideon to see if there's a Dropbox contributor's agreement with the PSF on file).

[ddfisher]

I'm pretty sure we checked that a while ago (and there was). Maybe someone needs to verify that for my account on bugs.python.org?

[gvanrossum]

OK, I've set that flag for your account.

[ddfisher]

Thanks! Double checked my email: yes Dropbox signed a CLA, and we should expect this to be covered.