use site variance with `in`/`out` modifiers

```py
a = [1]
b: list[Out[object]] = a # no error
```

```py
class A(Generic[T]):
def foo(self, t: T): ...
def foo(a: A[out[int]]):
a.foo(0) # error

foo(A[bool]()) # no error
```

I wonder if there is potential for an invariant use site variance modifier:
```py
class A(Generic[T_co]):
def foo(self): ..
def bar(self, t: Invariant[T_co]): ...

a1: A[int]
a1.foo()
a1.bar(1) # error
a2: A[Invariant[int]]
a2.foo()
a2.bar(1) # no error
```

The issue provides Python typing examples but names no files, tests, or implementation entry points. Start by locating the existing handling for `in`/`out` modifiers and use-site variance, then determine whether invariant modifiers are intended and how their behavior should be specified and tested.

Written by the indexing model from the issue text.