Add doctest flags to setup.cfg

[TomAugspurger]

pytest let's you add common doctest settings to the setup.cfg. I think we should add

doctest_optionflags= NORMALIZE_WHITESPACE

Any reason not to do this? Any others we should add?

[jorisvandenbossche]

Yes, I think that is a good idea.

[TomAugspurger]

Closed by #20284