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 :)

Riproduci il problema con gli esempi in t2.py ed esegui mypy sul file per confermare le due segnalazioni relative a hash. Traccia il modo in cui i corpi delle classi NamedTuple distinguono i campi dalle assegnazioni di metodi, quindi fai in modo che l’assegnazione superi il controllo dei tipi preservando la validazione delle dichiarazioni di campo non valide.