Fix instance vs callable subtyping/assignability

[carljm]

Summary

Fix some issues with subtying/assignability for instances vs callables. We need to look up dunders on the class, not the instance, and we should limit our logic here to delegating to the type of __call__, so it doesn't get out of sync with the calls we allow.

Also, we were just entirely missing assignability handling for __call__ implemented as anything other than a normal bound method (though we had it for subtyping.)

A first step towards considering what else we want to change in astral-sh/ty#491

Test Plan

mdtests

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

strawberry (https://github.com/strawberry-graphql/strawberry)
- error[invalid-assignment] strawberry/federation/scalar.py:133:9: Object of type `_T | None` is not assignable to `((...) -> Unknown) | None`
- error[invalid-argument-type] strawberry/types/scalar.py:110:29: Argument to bound method `__init__` is incorrect: Expected `(Any, /) -> Any`, found `_T`
- error[invalid-assignment] strawberry/types/scalar.py:209:9: Object of type `_T | None` is not assignable to `((...) -> Unknown) | None`
- Found 436 diagnostics
+ Found 433 diagnostics

jinja (https://github.com/pallets/jinja)
- error[invalid-argument-type] src/jinja2/ext.py:168:27: Argument to bound method `call` is incorrect: Expected `(...) -> Any`, found `Any | Undefined`
- Found 234 diagnostics
+ Found 233 diagnostics