[red-knot] Add support for typing.ClassVar (#15550)

## Summary

Add support for `typing.ClassVar`, i.e. emit a diagnostic in this
scenario:
```py
from typing import ClassVar

class C:
    x: ClassVar[int] = 1

c = C()
c.x = 3  # error: "Cannot assign to pure class variable `x` from an instance of type `C`"
```

## Test Plan

- New tests for the `typing.ClassVar` qualifier
- Fixed one TODO in `attributes.md`

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -169,6 +169,10 @@ class C:
     pure_class_variable1: ClassVar[str] = "value in class body"
     pure_class_variable2: ClassVar = 1
 
+    def method(self):
+        # TODO: this should be an error
+        self.pure_class_variable1 = "value set through instance"
+
 reveal_type(C.pure_class_variable1)  # revealed: str
 
 # TODO: this should be `Literal[1]`, or `Unknown | Literal[1]`.
@@ -182,7 +186,7 @@ reveal_type(c_instance.pure_class_variable1)  # revealed: str
 # TODO: Should be `Unknown | Literal[1]`.
 reveal_type(c_instance.pure_class_variable2)  # revealed: Unknown
 
-# TODO: should raise an error. It is not allowed to reassign a pure class variable on an instance.
+# error: [invalid-attribute-access] "Cannot assign to ClassVar `pure_class_variable1` from an instance of type `C`"
 c_instance.pure_class_variable1 = "value set on instance"
 
 C.pure_class_variable1 = "overwritten on class"

crates/red_knot_python_semantic/resources/mdtest/type_qualifiers/classvar.md

@@ -0,0 +1,93 @@
+# `typing.ClassVar`
+
+[`typing.ClassVar`] is a type qualifier that is used to indicate that a class variable may not be
+written to from instances of that class.
+
+This test makes sure that we discover the type qualifier while inferring types from an annotation.
+For more details on the semantics of pure class variables, see [this test](../attributes.md).
+
+## Basic
+
+```py
+from typing import ClassVar, Annotated
+
+class C:
+    a: ClassVar[int] = 1
+    b: Annotated[ClassVar[int], "the annotation for b"] = 1
+    c: ClassVar[Annotated[int, "the annotation for c"]] = 1
+    d: ClassVar = 1
+    e: "ClassVar[int]" = 1
+
+reveal_type(C.a)  # revealed: int
+reveal_type(C.b)  # revealed: int
+reveal_type(C.c)  # revealed: int
+# TODO: should be Unknown | Literal[1]
+reveal_type(C.d)  # revealed: Unknown
+reveal_type(C.e)  # revealed: int
+
+c = C()
+
+# error: [invalid-attribute-access]
+c.a = 2
+# error: [invalid-attribute-access]
+c.b = 2
+# error: [invalid-attribute-access]
+c.c = 2
+# error: [invalid-attribute-access]
+c.d = 2
+# error: [invalid-attribute-access]
+c.e = 2
+```
+
+## Conflicting type qualifiers
+
+We currently ignore conflicting qualifiers and simply union them, which is more conservative than
+intersecting them. This means that we consider `a` to be a `ClassVar` here:
+
+```py
+from typing import ClassVar
+
+def flag() -> bool:
+    return True
+
+class C:
+    if flag():
+        a: ClassVar[int] = 1
+    else:
+        a: str
+
+reveal_type(C.a)  # revealed: int | str
+
+c = C()
+
+# error: [invalid-attribute-access]
+c.a = 2
+```
+
+## Too many arguments
+
+```py
+class C:
+    # error: [invalid-type-form] "Type qualifier `typing.ClassVar` expects exactly one type parameter"
+    x: ClassVar[int, str] = 1
+```
+
+## Illegal `ClassVar` in type expression
+
+```py
+class C:
+    # error: [invalid-type-form] "Type qualifier `typing.ClassVar` is not allowed in type expressions (only in annotation expressions)"
+    x: ClassVar | int
+
+    # error: [invalid-type-form] "Type qualifier `typing.ClassVar` is not allowed in type expressions (only in annotation expressions)"
+    y: int | ClassVar[str]
+```
+
+## Used outside of a class
+
+```py
+# TODO: this should be an error
+x: ClassVar[int] = 1
+```
+
+[`typing.classvar`]: https://docs.python.org/3/library/typing.html#typing.ClassVar