Avoid reporting superfluous-parens after an is not operator

[timmartin]

Type of Changes

	Type
✓	🐛 Bug fix

Description

The superfluous-parens checker was incorrectly reporting that parentheses after an is not expression were superfluous. The checker seemed to be assuming that any expression prefixed with not was a unary expression and not part of a larger expression that might need parentheses for correctness.

Closes #5930

[coveralls]

Pull Request Test Coverage Report for Build 2234763611

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 252 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.02%) to 95.12%

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Files with Coverage Reduction	New Missed Lines	%
pylint/checkers/newstyle.py	1	96.15%
pylint/extensions/private_import.py	1	99.15%
pylint/lint/run.py	1	83.96%
pylint/checkers/design_analysis.py	2	98.9%
pylint/checkers/utils.py	2	95.76%
pylint/checkers/exceptions.py	5	97.7%
pylint/checkers/logging.py	8	94.29%
pylint/reporters/text.py	8	85.61%
pylint/checkers/base_checker.py	11	90.0%
pylint/checkers/format.py	11	96.21%

Totals
Change from base Build 2215661832:	-0.02%
Covered Lines:	15749
Relevant Lines:	16557

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💛 - Coveralls

[Pierre-Sassoulas]

LGTM