Selectors inside :not() not correctly parsed

[MichaelRoosz]

 .test:not(.a, .b) { display: none; }

will be parsed as:

Sabberworm\CSS\RuleSet\DeclarationBlock -> getSelectors():
  Sabberworm\CSS\Property\Selector '.test:not(.a',
  Sabberworm\CSS\Property\Selector '.b)'

[westonruter]

This has come up before. See #138

[ttk]

Also affects .test:is(.a, .b) { display: none; }