[mypyc] Use `method_sig` to get the method signature (#21567)

Fixes #21566

`create_ne_from_eq()` checks whether the class has an `__eq__` method
using `cls.has_method("__eq__")`, but `gen_glue_ne_method()` then
requires `cls.get_method("__eq__")` to return a concrete `FuncIR`.

In this package-cycle case, the inherited method declaration exists, but
the concrete `FuncIR` body is not available yet. This makes
`cls.has_method("__eq__")` true while `cls.get_method("__eq__")` returns
None.