[ty] Treat captured bound-method receivers covariantly (#28180)
## Summary
Previously, we compared a bound method's captured receiver
contravariantly. We now allow the same method bound to a subclass to be
a subtype of the method bound to its base class:
```python
from ty_extensions import static_assert
from ty_extensions._internal import TypeOf, is_subtype_of
class Base:
def method(self) -> int: ...
class Child(Base): ...
def check(base: Base, child: Child):
# Before: failed assertion. After: passes.
static_assert(is_subtype_of(TypeOf[child.method], TypeOf[base.method]))
```
The receiver exposed by `__self__` is an already-captured value, so it
is covariant. We also compare the bound signatures, so `Self` in a
remaining parameter still follows ordinary callable contravariance.
Classmethods follow the same rules.
