Remove post-release tags on setuptools' own build.

[jaraco]

Ref #4530

Summary of changes

Remove the post release tags for a cleaner build with the intention of later adopting setuptools_scm for version inference.

No news fragment is needed as it doesn't affect official releases.

Pull Request Checklist

• [ ] ~~Changes have tests~~
• [ ] News fragment added in newsfragments/. (See documentation for details)

[jaraco]

The docs build is failing on main.

[jaraco]

On further consideration, maybe this change does deserve a news fragment. Integrators probably have to enact the same workaround to convert the in-dev version to a released version, and that step will no longer be necessary after this change.