Fix false positive checking a class against type[Protocol] with instance attributes

[AnayGarodia]

Fixes #499

Checking a class (not an instance) against type[SomeProtocol] raised a false-positive TypeCheckError when the protocol declared a non-ClassVar attribute that the class sets in __init__:

class Proto(Protocol):
    foo: str

class Foo:
    def __init__(self) -> None:
        self.foo = "bar"

check_type(Foo, type[Proto])   # raised, though mypy/pyright accept it

Per PEP 544, an attribute annotation without ClassVar describes an instance attribute, so a class object is not expected to carry it. This implements the approach you outlined in the issue: when the subject is a class, check_protocol now ignores non-ClassVar attribute annotations and only requires the ClassVar ones to be present on the class. Instance checks are unchanged, and method/callable checks are untouched.

Added two regression tests: a class satisfying a protocol via an instance attribute no longer raises, and a class missing a required ClassVar member still does. The full tests/test_checkers.py suite passes.

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[coveralls]

coverage: 94.855% (+0.008%) from 94.847% — AnayGarodia:fix-protocol-class-instance-attrs into agronholm:master

[agronholm]

Thanks!