Fix incorrect handling of orphan fragment names consists of only digits

[SmileyChris]

Fixes #584

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure local test run is green.
• Create a file in src/towncrier/newsfragments/. Describe your
  change and include important information. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).

[SmileyChris]

Looking closer the true source of the error is because strip_if_integer_string reads the orphan of +12345 as an actual integer, stripping off the '+', causing this issue to lose its status as an orphan!

[SmileyChris]

There, that's tidier!

[adiroiban]

Looks good. Many thanks. Only a minor comment regarding the release notes.

[webknjaz]

Could you add another non-orphan test with an identifier looking like a commit sha? In the templates I've started using recently, I try to output PRs/issues, commits and arbitrary identifiers separately: https://github.com/cherrypy/cheroot/blob/3591a1c/docs/changelog-fragments.d/.towncrier-template.rst.j2#L59-L63.

It'd be nice if that remained functional. Hence an explicit test...

[SmileyChris]

I'd say that should be a separate PR -- that's an interesting functional test but not really tied to the changes being made here.

[webknjaz]

The reason I brought it up is that I wasn't sure if this PR influences it. So it's kinda related as a possible regression. But I understand that it may be non-atomic and separate otherwise.

[adiroiban]

Thanks for your feedback.

This test is named test_orphan_all_digits . Based on that, I think that +123 is ok.

I also think that using a git commit ref is also a valid test... but it should be a separate test.

[adiroiban]

It looks good. I have enabled auto-merge.

Thanks for your help.

I am happy to review and approve a PR that add more tests... but that should not be a blocker for this PR.