Adopt setuptools_scm

[jaraco]

• Replace static version with setuptools-scm version.
• Remove bumpversion and rely on jaraco.develop for finalize operation.
• Vendor setuptools_scm instead of declaring it to avoid bootstrapping issue.

Summary of changes

Closes #4530

Pull Request Checklist

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

[jaraco]

I don't understand the test failures. The tests pass for me locally. Also, the test failures seem largely unrelated to the change. The only thing I can think is that maybe the tests are somehow reliant on Setuptools' own MANIFEST.in. I'm going to sleep on it and tomorrow try to replicate the issue in a Docker image or using the act tool.

[abravalheri]

Does excluding this file also mean that we no longer need to repair the changelog and that #2666 is already solved?

[jaraco]

Maybe so. I don't encounter the issue in other skeleton projects, so I assume so. If it turns out it's still a problem, I'll just strip out whatever customizations setuptools is using that introduce the bug. No reason to carry around this debt.

[jaraco]

I just noticed that this line gets moved, but it was already present. I'll move it back.