DOC/TST: Added NORMALIZE_WHITESPACE flag (#20284)

* DOC/TST: Added NORMALIZE_WHITESPACE flag

I think we want this to be always on. You can disable it with

```
doctest: -NORMALIZE_WHITESPACE
```

if necessary.

* Added IGNORE_EXCEPTION_DETAIL for consistency

Author: TomAugspurger

setup.cfg

@@ -32,3 +32,4 @@ markers =
     slow: mark a test as slow
     network: mark a test as network
     high_memory: mark a test as a high-memory only
+doctest_optionflags= NORMALIZE_WHITESPACE IGNORE_EXCEPTION_DETAIL