non-string fieldnames for csv.DictReader and csv.DictWriter

[Akuli]

Fixes #4800

To check that this works:

import csv
import enum
from typing import TYPE_CHECKING

class Foo(enum.Enum):
    X = 1
    Y = 2

def main() -> None:
    with open('foo.csv', 'w') as file:
        writer = csv.DictWriter(file, list(Foo))
        writer.writeheader()
        writer.writerow({Foo.X: "lol", Foo.Y: "wat"})

    with open('foo.csv', 'r') as file:
        foo_reader = csv.DictReader(file, list(Foo))
        print(list(foo_reader))
        if TYPE_CHECKING:
            reveal_type(next(foo_reader))

    with open('foo.csv', 'r') as file:
        string_reader = csv.DictReader(file)
        print(list(string_reader))
        if TYPE_CHECKING:
            reveal_type(next(string_reader))

main()

Output:

[{Foo.X: 'X', Foo.Y: 'Y'}, {Foo.X: 'lol', Foo.Y: 'wat'}]
[{'X': 'lol', 'Y': 'wat'}]

mypy on master:

foo.py:13: error: Dict entry 0 has incompatible type "Foo": "str"; expected "str": "Any"
foo.py:13: error: Dict entry 1 has incompatible type "Foo": "str"; expected "str": "Any"
foo.py:19: note: Revealed type is "builtins.dict*[builtins.str, builtins.str]"
foo.py:25: note: Revealed type is "builtins.dict*[builtins.str, builtins.str]"
Found 2 errors in 1 file (checked 1 source file)

mypy after this PR:

foo.py:19: note: Revealed type is "builtins.dict*[foo.Foo*, builtins.str]"
foo.py:25: note: Revealed type is "builtins.dict*[builtins.str*, builtins.str]"

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉