[ty] Avoid enforcing `__new__` with custom metaclasses (#25180)
## Summary
This PR makes enum member validation account for custom
`EnumMeta.__new__` methods. Prior to this change, we validated the raw
right-hand side of an enum member against inherited `_value_,`
`__new__,` or `__init__`. But that's too strict for Django’s
`IntegerChoices` pattern...
```python
from django.db import models
class Status(models.IntegerChoices):
GOOD = 1, "I like this"
```
Django’s custom metaclass strips the label out of the member value
before `IntEnum` construction, so the raw `(1,
"I like this")` tuple is _not_ the value that should be checked against
`int.__new__`.
We now detect a custom enum metaclass `__new__` and treat member values
as potentially transformed. In that case, we skip raw-RHS validation
against enum construction hooks, and `.value` / `._value_` falls back to
`Any` (unless the enum class itself declares an explicit `_value_`
annotation).
Pyright has similar behavior here:
https://github.com/microsoft/pyright/blob/63998f4d0720a86447f4c4a04716e34f3e703660/packages/pyright-internal/src/analyzer/enums.ts#L653-L660
But we do differ in this case:
```python
class Status(IntegerChoices):
_value_: int
GOOD = 1, "label"
```
Pyright exposes `Status.GOOD.value` as `int`, but still reports that the
raw tuple is not assignable to `int`. We expose `Status.GOOD.value` as
`int`, and don't report anything.
Closes https://github.com/astral-sh/ty/issues/3468.
