PEP 448: Fix misleading passage in the abstract

[MrMino]

The passage in the PEP misleads its readers into believing, that unpacking in comprehensions is supported. This is not the case, as presented by the abstract itself:

This PEP does not include unpacking operators inside list, set and dictionary comprehensions although this has not been ruled out for future proposals.

See discussion in https://discuss.python.org/t/pep-448-contradictory-abstract/40657/6

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[cpython-cla-bot]

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[Rosuav]

Saw this mentioned on Discourse, and I broadly agree, but I would be inclined to do the edit at the end of the abstract instead - it looks like maybe that sentence is left over from an earlier edit, possibly.

Would like to get input from the PEP author here.

[CAM-Gerlach]

Would like to get input from the PEP author here.

@Veedrac ?

[gvanrossum]

I approve of fixing this (using either the OP's wording or CAM's suggestion). It seems this was an oversight introduced when (in commit 401c977) I accepted most of the original PEP but rejected unpacking in compehensions and generator expressions. I simply failed to update that sentence in the opening paragraph of the Abstract.

[Veedrac]

@CAM-Gerlach You have my retroactive endorsement.