Don't emit unsubscriptable-object for string annotations

[cdce8p]

Description

We previously checked if a class is explicitly mentioned in PEP 585 as being subscriptable. That frequently results in errors as some classes are only generic in type stubs. We could continue to update the set of subscriptable names, but that still wouldn't cover custom packages and is impractical.

With this PR, pylint will ignore all string annotations (explicit or via from __future__ import annotations). We might miss a few issues, however those aren't runtime errors and a type checker is likely used anyway witch would catch these.

Not emitting false-positives is more important than catching all errors here IMO.

Closes #4369
Closes #6523

[coveralls]

Pull Request Test Coverage Report for Build 2286115814

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 95.314%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pylint/checkers/utils.py	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
pylint/checkers/utils.py	1	95.54%

Totals
Change from base Build 2283754721:	-0.01%
Covered Lines:	15986
Relevant Lines:	16772

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💛 - Coveralls

[Pierre-Sassoulas]

Agree with the sentiment about reducing false positives vs false negative.