[red-knot] instance attributes

[carljm]

• Write test suite with desired behavior (resolved by [red-knot] Initial tests for instance attributes #15474 and [red-knot] Initial tests for instance attributes #15474)
• Support pure instance variables, declared in class body (resolved by [red-knot] Pure instance variables declared in class body #15515)
• Add support for typing.ClassVar (resolved by [red-knot] Add support for typing.ClassVar #15550)
• Raise diagnostics for invalid assignments to attributes (see [red-knot] Out-of-body (re-)definitions of class variables #15456; resolved by [red-knot] Invalid assignments to attributes #15613, [red-knot] Add test for nested attribute access #15684 and [red-knot] Improved error message for attribute-assignments #15668)
• Infer Unknown | T_inferred for undeclared class variables (resolved by [red-knot] Use Unknown | T_inferred for undeclared public symbols #15674)
• Support implicit instance attributes, which are not defined in the class body, only in __init__ or another method (resolved by [red-knot] Implicit instance attributes #15811)
• [red-knot] Support unpacking attribute assignments: self.x, self.y = … #15962
• [red-knot] Support implicit class attributes (only defined in a @classmethod) #15965
• [red-knot] Attribute assignments in @staticmethods should not influence instance attributes #15961
• Handle cycles in type inference once we understand the type of self
• [red-knot] Attributes implicitly declared via their parameter types #15960
• [red-knot] Descriptor protocol #15966
• [red-knot] Diagnostic for class-object access to pure instance variables #15963
• [red-knot] Diagnostic for conflicting declared attribute types #15964
• [red-knot] Detect unreachable attributes assignments #15967
• Implement method call checking (no ticket yet, blocked on descriptor support)
• [red-knot] Bare ClassVar annotations #15767

See also: Markdown test suite with some open TODOs: https://github.com/astral-sh/ruff/blob/main/crates/red_knot_python_semantic/resources/mdtest/attributes.md

[sharkdp]

Modified the description to reflect the current progress