Allow calling dict methods on `TypedDict`

[grievejia]

Minimal repro:

from typing import TypedDict

class D(TypedDict):
    x: int

def test(d: D) -> None:
    d.copy()

Expected: No error
Actual: Object of class Mapping has no attribute copy [missing-attribute]

[yangdanny97]

This one is a bit odd, since the typing spec says TypedDict is assignable to Mapping but not Dict, so for attribute lookup we just treat TypedDict as mapping.

I wonder if we could fix this by just making TypedDict be treated as a dict when we look up attributes. We might need to ban some operatons like clear() tho

https://typing.python.org/en/latest/spec/typeddict.html#supported-and-unsupported-operations