NamedTuple subclasses do not pick up __new__'s signature

[toolforger]

mypy inspects the field names instead of looking at the signature of __new__.
(I didn't try __init__, that wasn't my use case.)

Mypy 0.610, Python 3.6.3.
Not tried Mypy from git master (haven't checked how to do that).

Code:

#! /usr/bin/env python3

from typing import List, NamedTuple

class Command(NamedTuple):
    subcommands: List[str]

class HelpCommand(Command):
    def __new__(cls):
        return Command.__new__(cls, subcommands=[])

HelpCommand() # fails in mypy, works in Python
HelpCommand(subcommands=[]) # works in mypy, fails in Python

Command: python3 -m mypy typetest.py

Output:

typetest.py:12: error: Too few arguments for "HelpCommand"

Python has a different idea about what should work and what shouldn't:

$ ./typetest.py 
Traceback (most recent call last):
  File "./typetest.py", line 12, in <module>
    HelpCommand(subcommands=[]) # works
TypeError: __new__() got an unexpected keyword argument 'subcommands'

[ethanhs]

Closing as a duplicate of #1279.