bpo-19610: Warn if distutils is provided something other than a list to some fields (#4685)

* Rather than raise TypeError, warn and call list() on the value.

* Fix tests, revise NEWS and whatsnew text.

* Revise documentation, a string is okay as well.

* Ensure 'requires' and 'obsoletes' are real lists.

* Test that requires and obsoletes are turned to lists.