[issue40716] Pegen: improve error messages for unparenthesized from imports with trailing comma

[Lysandros Nikolaou]

Change by Lysandros Nikolaou <lisandrosnik at gmail.com>:


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nosy: +gvanrossum, lys.nikolaou, pablogsal

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40716>
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