gh-105858: Expose some union-related objects as internal APIs

[JelleZijlstra]

We now use these in the AST parsing code after gh-105880. A few comparable types (e.g.,
NoneType) are already exposed as internal APIs.

• Issue: Improve the constructors of AST nodes #105858

[JelleZijlstra]

This should fix these buildbot failures: #105880 (comment)

I tested locally by running ./python.exe -m test -u cpu test_peg_generator -v.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @JelleZijlstra for commit 5142722 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[JelleZijlstra]

The macOS builds are failing with something related to 2to3 that doesn't seem due to my changes:

Error: The `brew link` step did not complete successfully
The formula built, but is not symlinked into /usr/local
Could not symlink bin/2to3
Target /usr/local/bin/2to3
already exists. You may want to remove it:
  rm '/usr/local/bin/2to3'

To force the link and overwrite all conflicting files:
  brew link --overwrite python@3.12

To list all files that would be deleted:
  brew link --overwrite python@3.12 --dry-run