Include pyproject.toml in sdist

[daa]

Summary of changes

Include pyproject.toml in source distribution if it is present.
Closes #1632

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details

[jaraco]

Gah. This PR seemed like a pretty innocuous request, and since the tests had passed, I merged it. But now I see this change causes tests to fail in master when pip 19 is present (now that setuptools' own pyproject.toml is now present). This is another manifestation of #1644.

[pganssle]

@jaraco Is this included in a release yet?

I think this shouldn't be included in any release unless that release also handles the "default PEP 517 backend" issue.

[jaraco]

It is not.

[jaraco]

This merge is the last thing on master. I'm going to force-push master back to its prior state.

[jaraco]

setuptools master $ git push -f origin 97e8a:master                                                                                                                                      
Total 0 (delta 0), reused 0 (delta 0)
To https://github.com/pypa/setuptools
 + 7a4a99a3...97e8ad4f 97e8a -> master (forced update)

[jaraco]

If you'd pulled the latest master, please on master: git fetch origin master; git reset --hard origin/master.