[ty] Make TypeIs invariant in its type argument (#20428)

## Summary

What it says on the tin. See the [typing
spec](https://docs.python.org/3/library/typing.html#typing.TypeIs) for
justification.

## Test Plan

Add more tests to PEP 695 `variance.md` suite.

Author: ericmarkmartin

crates/ty_python_semantic/resources/mdtest/generics/pep695/variance.md

@@ -752,6 +752,36 @@ b_container = ClassContainer[B](B)
 a_instance: A = use_a_class_container(b_container)  # This should work
 ```
 
+## TypeIs
+
+```toml
+[environment]
+python-version = "3.13"
+```
+
+`TypeIs[T]` is invariant in `T`. See the [typing spec][typeis-spec] for a justification.
+
+```py
+from typing import TypeIs
+from ty_extensions import is_assignable_to, is_subtype_of, static_assert
+
+class A:
+    pass
+
+class B(A):
+    pass
+
+class C[T]:
+    def check(x: object) -> TypeIs[T]:
+        # this is a bad check, but we only care about it type-checking
+        return False
+
+static_assert(not is_subtype_of(C[B], C[A]))
+static_assert(not is_subtype_of(C[A], C[B]))
+static_assert(not is_assignable_to(C[B], C[A]))
+static_assert(not is_assignable_to(C[A], C[B]))
+```
+
 ## Inheriting from generic classes with inferred variance
 
 When inheriting from a generic class with our type variable substituted in, we count its occurrences
@@ -837,3 +867,4 @@ static_assert(is_subtype_of(DerivedContravariant[A], DerivedContravariant[B]))
 
 [linear-time-variance-talk]: https://www.youtube.com/watch?v=7uixlNTOY4s&t=9705s
 [spec]: https://typing.python.org/en/latest/spec/generics.html#variance
+[typeis-spec]: https://typing.python.org/en/latest/spec/narrowing.html#typeis

crates/ty_python_semantic/src/types.rs

@@ -6620,6 +6620,7 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
                 .map(|ty| ty.variance_of(db, typevar))
                 .collect(),
             Type::SubclassOf(subclass_of_type) => subclass_of_type.variance_of(db, typevar),
+            Type::TypeIs(type_is_type) => type_is_type.variance_of(db, typevar),
             Type::Dynamic(_)
             | Type::Never
             | Type::WrapperDescriptor(_)
@@ -6640,7 +6641,6 @@ impl<'db> VarianceInferable<'db> for Type<'db> {
             | Type::BoundSuper(_)
             | Type::TypeVar(_)
             | Type::NonInferableTypeVar(_)
-            | Type::TypeIs(_)
             | Type::TypedDict(_)
             | Type::TypeAlias(_) => TypeVarVariance::Bivariant,
         };
@@ -10956,6 +10956,16 @@ impl<'db> TypeIsType<'db> {
     }
 }
 
+impl<'db> VarianceInferable<'db> for TypeIsType<'db> {
+    // See the [typing spec] on why `TypeIs` is invariant in its type.
+    // [typing spec]: https://typing.python.org/en/latest/spec/narrowing.html#typeis
+    fn variance_of(self, db: &'db dyn Db, typevar: BoundTypeVarInstance<'db>) -> TypeVarVariance {
+        self.return_type(db)
+            .with_polarity(TypeVarVariance::Invariant)
+            .variance_of(db, typevar)
+    }
+}
+
 /// Walk the MRO of this class and return the last class just before the specified known base.
 /// This can be used to determine upper bounds for `Self` type variables on methods that are
 /// being added to the given class.