[ty] Infer single-valuedness for enums based on `int`/`str` (#19510)
## Summary
We previously didn't recognize `Literal[Color.RED]` as single-valued, if
the enum also derived from `str` or `int`:
```py
from enum import Enum
class Color(str, Enum):
RED = "red"
GREEN = "green"
BLUE = "blue"
def _(color: Color):
if color == Color.RED:
reveal_type(color) # previously: Color, now: Literal[Color.RED]
```
The reason for that was that `int` and `str` have "custom" `__eq__` and
`__ne__` implementations that return `bool`. We do not treat enum
literals from classes with custom `__eq__` and `__ne__` implementations
as single-valued, but of course we know that `int.__eq__` and
`str.__eq__` are well-behaved.
## Test Plan
New Markdown tests.
ruff
5a55bab3 - [ty] Infer single-valuedness for enums based on `int`/`str` (#19510)
