[1.20 regression] False positive for checking reachability on dict with single-variant enum keys

[kmurphy4]

Bug Report
After upgrading from 1.19.x to 1.20.x, I'm seeing a false positive with reachability checking for dicts with single-variant Enum keys.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.12&flags=warn-unreachable&gist=3ba2a6f89ea69ebf0ce78f0d91fb0e7f

import enum

class Color(enum.Enum):
    RED = enum.auto()

def foo(colors: list[Color]) -> None:
    counts: dict[Color, int] = {}
    for color in colors:
        if color not in counts:
            counts[color] = 0  # error: Statement is unreachable  [unreachable]
        counts[color] += 1

If I add a second Enum variant, the error goes away.

Interestingly, the error also goes away if I remove the explicit type annotation.

Expected Behavior

With v1.19.1, I get no errors.

[hauntsaninja]

Thanks, fixed in #21411