Issue #12947: Better workaround for the problem with doctest directives being stripped from code examples that are intended to illustrate those directives

gil_interval == 1 in extreme situations.

Patch by Serhiy Storchaka.

The default reST role `foo` can now be used in NEWS to refer to Python objects. Use it for a fraction of the current NEWS file.

* rename to "Changelog"
* recognize more issue links
* add NEWS as a dependency for Sphinx

I went through all the versionchanged/versionadded tags in the os doc page for
this changeset.

Optimizations suggested by Serhiy Storchaka.