Add __new__ to old ast.Constant aliases

[srittau]

This aligns the old constant classes closer to the actual implementation. It's not perfect, as it doesn't replicate the __instancecheck__ and doesn't replicate the fact that theoretical sub-classes of the old constant classes are still possible and don't get replaced with Constant.

[srittau]

The only primer hit is (as expected) in a piece of code that won't work on Python 3.14, but can actually be simplified.

[srittau]

This aligns the old constant classes closer to the actual implementation. It's not perfect, as it doesn't replicate the __instancecheck__ and doesn't replicate the fact that theoretical sub-classes of the old constant classes are still possible and don't get replaced with Constant.

That said, the minimal primer fallout and the fact that these are deprecated classes, so no new code will use them, indicates that this is not a problem.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spack (https://github.com/spack/spack)
+ lib/spack/spack/cmd/style.py:666: error: Incompatible types in assignment (expression has type "str | bytes | int | float | complex | EllipsisType | None", variable has type "str")  [assignment]