Skip symlinks when creating SBOM for source tarball

[hugovk]

Building an SBOM for artifact 'Python-3.14.0a7.tgz'
💥  Building SBOM artifacts
Traceback (most recent call last):
  File "/Users/hugo/github/release-tools/run_release.py", line 1388, in <module>
    main()
  File "/Users/hugo/github/release-tools/run_release.py", line 1384, in main
    automata.run()
  File "/Users/hugo/github/release-tools/run_release.py", line 248, in run
    raise e from None
  File "/Users/hugo/github/release-tools/run_release.py", line 245, in run
    self.current_task(self.db)
  File "/Users/hugo/github/release-tools/release.py", line 126, in __call__
    return getattr(self, "function")(db)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/hugo/github/release-tools/run_release.py", line 656, in build_sbom_artifacts
    sbom_data = sbom.create_sbom_for_source_tarball(tarball_path)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/hugo/github/release-tools/sbom.py", line 616, in create_sbom_for_source_tarball
    assert member.isfile() and member.name.startswith(f"Python-{cpython_version}/")
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

That's because Python-3.14.0a7/Misc/mypy/_colorize.py is a symlink and member.isfile() is False.

Because https://github.com/python/cpython/tree/main/Misc/mypy now has symlinks, added in python/cpython#131509.

Let's skip them.

[sethmlarson]

LGTM!