[red-knot] Assignability for subclasses of `Any` and `Unknown` (#17557)

## Summary

Allow (instances of) subclasses of `Any` and `Unknown` to be assignable
to (instances of) other classes, unless they are final. This allows us
to get rid of ~1000 false positives, mostly when mock-objects like
`unittest.mock.MagicMock` are assigned to various targets.

## Test Plan

Adapted and new Markdown tests.

crates/red_knot_python_semantic/resources/mdtest/annotations/any.md

@@ -50,10 +50,9 @@ y: Any = "not an Any"  # error: [invalid-assignment]
 
 The spec allows you to define subclasses of `Any`.
 
-TODO: Handle assignments correctly. `Subclass` has an unknown superclass, which might be `int`. The
+`Subclass` has an unknown superclass, which might be `int`. The assignment to `x` should not be
-assignment to `x` should not be allowed, even when the unknown superclass is `int`. The assignment
+allowed, even when the unknown superclass is `int`. The assignment to `y` should be allowed, since
-to `y` should be allowed, since `Subclass` might have `int` as a superclass, and is therefore
+`Subclass` might have `int` as a superclass, and is therefore assignable to `int`.
-assignable to `int`.
 
 ```py
 from typing import Any
@@ -63,13 +62,33 @@ class Subclass(Any): ...
 reveal_type(Subclass.__mro__)  # revealed: tuple[Literal[Subclass], Any, Literal[object]]
 
 x: Subclass = 1  # error: [invalid-assignment]
-# TODO: no diagnostic
+y: int = Subclass()
-y: int = Subclass()  # error: [invalid-assignment]
 
 def _(s: Subclass):
     reveal_type(s)  # revealed: Subclass
 ```
 
+`Subclass` should not be assignable to a final class though, because `Subclass` could not possibly
+be a subclass of `FinalClass`:
+
+```py
+from typing import final
+
+@final
+class FinalClass: ...
+
+f: FinalClass = Subclass()  # error: [invalid-assignment]
+```
+
+A use case where this comes up is with mocking libraries, where the mock object should be assignable
+to any type:
+
+```py
+from unittest.mock import MagicMock
+
+x: int = MagicMock()
+```
+
 ## Invalid
 
 `Any` cannot be parameterized:

crates/red_knot_python_semantic/resources/mdtest/mdtest_custom_typeshed.md

@@ -22,6 +22,7 @@ We can then place custom stub files in `/typeshed/stdlib`, for example:
 `/typeshed/stdlib/builtins.pyi`:
 
 ```pyi
+class object: ...
 class BuiltinClass: ...
 
 builtin_symbol: BuiltinClass

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -47,6 +47,13 @@ static_assert(is_assignable_to(Unknown, Literal[1]))
 static_assert(is_assignable_to(Any, Literal[1]))
 static_assert(is_assignable_to(Literal[1], Unknown))
 static_assert(is_assignable_to(Literal[1], Any))
+
+class SubtypeOfAny(Any): ...
+
+static_assert(is_assignable_to(SubtypeOfAny, Any))
+static_assert(is_assignable_to(SubtypeOfAny, int))
+static_assert(is_assignable_to(Any, SubtypeOfAny))
+static_assert(not is_assignable_to(int, SubtypeOfAny))
 ```
 
 ## Literal types

crates/red_knot_python_semantic/src/types/class.rs

@@ -324,8 +324,6 @@ impl<'db> ClassType<'db> {
     }
 
     pub(super) fn is_assignable_to(self, db: &'db dyn Db, other: ClassType<'db>) -> bool {
-        // `is_subclass_of` is checking the subtype relation, in which gradual types do not
-        // participate, so we should not return `True` if we find `Any/Unknown` in the MRO.
         if self.is_subclass_of(db, other) {
             return true;
         }
@@ -341,6 +339,16 @@ impl<'db> ClassType<'db> {
             }
         }
 
+        if self.iter_mro(db).any(|base| {
+            matches!(
+                base,
+                ClassBase::Dynamic(DynamicType::Any | DynamicType::Unknown)
+            )
+        }) && !other.is_final(db)
+        {
+            return true;
+        }
+
         false
     }