When a file is marked # sage.doctest: optional - xyz, we omit it from doctesting unless --optional=xyz is given.
This will save us from having to add lots of # optional - ... tags to files in the course of modularization (#29705)
We do this by extending sage.doctest.control.skipfile, which already parses files for # nodoctest file directives.
Previous related proposals/discussions: #3260, #20427
Also related: #30746
CC: @simon-king-jena @kiwifb @roed314 @saraedum @nthiery @videlec
Component: doctest framework
Keywords: sd111
Author: Matthias Koeppe, John Palmieri
Branch/Commit: 5cc1288
Reviewer: John Palmieri, Matthias Koeppe
Issue created by migration from https://trac.sagemath.org/ticket/30778