Fix repo check: b1 is special and still made from main, not 3.x

[hugovk]

Fix the check added in #334:

❯ .venv/bin/python run_release.py --repository ../cpython-3.15 --ssh-user hugovk --release 3.15.0b1
Release data:
- Branch: 3.15
- Release tag: 3.15.0b1
- Normalized release tag: 3.15.0
- Git repo: ../cpython-3.15
- SSH username: hugovk
- SSH key: Default
- Sign with GPG: False
- Security release: False

✅  Checking gh is available
✅  Checking Git is available
✅  Checking make is available
✅  Checking blurb is available
✅  Checking Docker is available
✅  Checking Docker is running
✅  Checking autoconf is available
✅  Validating ssh connection to downloads.nyc1.psf.io and docs.nyc1.psf.io
✅  Checking Sigstore CLI
💥  Checking CPython repository branch
Traceback (most recent call last):
  File "/Users/hugo/github/release-tools/run_release.py", line 1483, in <module>
    main()
    ~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 1479, in main
    automata.run()
    ~~~~~~~~~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 281, in run
    raise e from None
  File "/Users/hugo/github/release-tools/run_release.py", line 278, in run
    self.current_task(self.db)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/hugo/github/release-tools/release.py", line 151, in __call__
    return getattr(self, "function")(db)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/Users/hugo/github/release-tools/run_release.py", line 433, in check_cpython_repo_branch
    raise ReleaseException(
    ...<2 lines>...
    )
ReleaseException: CPython repository is on main branch, expected 3.15

Beta 1 is special: like all alphas, we start from the main branch. At the end of b1, the next 3.x branch is created, and then that will be used for b2 and later.

[savannahostrowski]

A stylistic nit which you can take or leave as is, but this looks right to me!

[hugovk]

Two new commits, followup bug when trying to make the new 3.15 branch:

✅  Merge the tag into the release branch
Already on 'main'
Your branch is ahead of 'origin/main' by 1 commit.
  (use "git push" to publish your local commits)
fatal: a branch named 'main' already exists
💥  Branch out new versions and prepare main branch
Traceback (most recent call last):
  File "/Users/hugo/github/release-tools/run_release.py", line 1483, in <module>
    main()
    ~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 1479, in main
    automata.run()
    ~~~~~~~~~~~~^^
  File "/Users/hugo/github/release-tools/run_release.py", line 281, in run
    raise e from None
  File "/Users/hugo/github/release-tools/run_release.py", line 278, in run
    self.current_task(self.db)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/Users/hugo/github/release-tools/release.py", line 151, in __call__
    return getattr(self, "function")(db)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/Users/hugo/github/release-tools/run_release.py", line 1247, in branch_new_versions
    subprocess.check_call(
    ~~~~~~~~~~~~~~~~~~~~~^
        ["git", "checkout", "-b", release_tag.branch],
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        cwd=db["git_repo"],
        ^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Library/Frameworks/Python.framework/Versions/3.14/lib/python3.14/subprocess.py", line 420, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['git', 'checkout', '-b', 'main']' returned non-zero exit status 128.

From here:

def branch_new_versions(db: ReleaseShelf) -> None:
    release_tag: release_mod.Tag = db["release"]

    if not release_tag.is_feature_freeze_release:
        return

    subprocess.check_call(["git", "checkout", "main"], cwd=db["git_repo"])

    subprocess.check_call(
        ["git", "checkout", "-b", release_tag.branch],
        cwd=db["git_repo"],
    )

This is a feature freeze, we don't return early. But the new branch should be x.y (basic_version), not main (release_tag.branch).

And a similar change in post_release_tagging: switch to x.y branch not main.