setuptools: fix regression with egg_info command

[benoit-pierre]

Ensure version is tagged only once. Fix #1462.

[jaraco]

Thank you, especially for the regression test. Nicely done.

[benoit-pierre]

Off-topic: I think the fact that I need to create the PR before I can update it with a towncrier entry (because I need the PR number) really annoying. Don't know how to improve the workflow though...

[pganssle]

@benoit-pierre Agreed, that's one of my biggest pet peeves with town crier.

For some background, town crier came out of Twisted, which has a much more formal process for contributions. For both Twisted and CPython, every PR must have an associated ticket in the bug tracker, and that's the number that's supposed to go in the changelog.

I think we were never quite so formal about it because we wanted people to be able to do drive-by PRs without an associated issue. If we want to alleviate the issue (which I agree is a pain), I see two ways forward:

1. All PRs must have an associated issue, and the news fragment is numbered by issue.
2. Change the workflow so that you can commit a file called #PR_STUB#.change.rst in the changelog.d, and we write a bot to update the PR when a number becomes available.