[issue26188] Provide more helpful error message when `await` is called inside non-`async` method

[SilentGhost]

Changes by SilentGhost <ghost.adh at gmail.com>:


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components: +asyncio
nosy: +giampaolo.rodola, gvanrossum, haypo, pitrou, yselivanov

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue26188>
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