Allow method assignment in body of type extending typing.NamedTuple

this currently works for non-namedtuple classes (both runtime and mypy-time):

class C:
    x: int

    __hash__ = object.__hash__

and it works for typing.NamedTuple at runtime:

from typing import NamedTuple

class C(NamedTuple):
    x: int

    __hash__ = object.__hash__

$ python3 -i t2.py
>>> c = C(1)
>>> hash(c)
-9223363269847169460
>>> object.__hash__(c)
-9223363269847169460
>>> tuple.__hash__(c)
3430019387558

but fails at type-checking time:

$ mypy --version && python --version --version
mypy 0.770
Python 3.6.8 (default, Oct  7 2019, 12:59:55) 
[GCC 8.3.0]
$ mypy t2.py
t2.py:7: error: NamedTuple field name cannot start with an underscore: __hash__
t2.py:7: error: Invalid statement in NamedTuple definition; expected "field_name: field_type [= default]"
Found 2 errors in 1 file (checked 1 source file)

concrete usecase: __hash__ is in the critical path (~13%) for some code I'm writing and the extra layer of function which calls into object.__hash__'s overhead is significant:

# mypy is ok with this
from typing import NamedTuple

class C(NamedTuple):
    x: int

    def __hash__(self) -> int:
        return object.__hash__(self)

I'd be happy to hack on this given some pointers :)

Reproduce el problema con los ejemplos de t2.py y ejecuta mypy en el archivo para confirmar los dos diagnósticos de hash. Sigue cómo los cuerpos de clase de NamedTuple distinguen los campos de las asignaciones de métodos y, después, haz que la asignación pase la comprobación de tipos manteniendo la validación de las declaraciones de campos no válidas.