TypedDict fields cannot be defined with variables #8186

Bug or opening a feature request?

A bug.

Minimal example

Using the alternative definition syntax for TypedDict (which should be equivalent according to PEP 589), the field names cannot be stored in variables (expected). But, I would expect Final or Literal typed variables to be accepted. See the following example:

from typing import TypedDict, Final, Literal

A = TypedDict("A", {"x": int})  # No issues, as expected.
d: A = {"x": 1}

x: Final = "x"
B = TypedDict("B", {x: int})  # Unexpected error 1.
dd: B = {"x": 1}  # Unexpected error 2.

x_literal: Literal["x"] = "x"
C = TypedDict("C", {x_literal: int})  # Unexpected error 3.
ddd: C = {"x": 1}  # Unexpected error 4.

What is the actual behaviour/output?

$ mypy repro.py
repro.py:8: error: Invalid TypedDict() field name
repro.py:9: error: Extra key 'x' for TypedDict "TypedDict"
repro.py:12: error: Invalid TypedDict() field name
repro.py:13: error: Extra key 'x' for TypedDict "TypedDict"
Found 4 errors in 1 file (checked 1 source file)

What is the behaviour/output you expect?

No errors, or at least one of the two options succeeding (Final or Literal).

The type annotations are equivalent, as far as I can tell.

>>> x: Final[str] = "x"
>>> A = TypedDict("A", {"x": int})
>>> A.__annotations__
{'x': <class 'int'>}
>>> B = TypedDict("B", {x: int})
>>> B.__annotations__
{'x': <class 'int'>}

What are the versions of mypy and Python you are using?

$  python --version
Python 3.8.0
$ mypy --version
mypy 0.761

Same behaviour on master (ce186f4).

Use case

We have defined constants for the field names of dictionaries read from JSON files. I would like to make TypedDict's for these various dictionaries so that mypy can better check our code (instead of marking all dicts with Dict[str, Any].

I would like to define the fields using these predefined constants.

# a.py
FIELD_A: Final = "field_a"
FIELD_B: Final = "field_b"

# b.py
MyDict = TypedDict("MyDict", {FIELD_A: int, FIELD_B: bool})