replace self.config with self.linter.config in class StringQuoteChecker

[4nds]

This PR fixes 4nds#1.

It resolves AttributeError: 'StringQuoteChecker' object has no attribute 'config' in

pylint-quotes/pylint_quotes/checker.py

Line 289 in c9aae00

preferred_quote = SMART_QUOTE_OPTS.get(self.config.string_quote)

Property config of StringQuoteChecker class was depreciated since Pylint 2.14.0,

The config attribute of BaseChecker has been deprecated. You can use checker.linter.config to access the global configuration object instead of a checker-specific object.

and later removed in Pylint 3.0.0.

This PR replaces self.config with self.linter.config in class StringQuoteChecker.