DOC: fix doctests for pandas/core/base.py for new string dtype

[paulbehrisch]

This is an attempt to fixe the failing doctests for pandas/core/base.py referenced in issue #61886

Test run

================================================================================================================================ test session starts =================================================================================================================================
platform darwin -- Python 3.13.5, pytest-8.4.1, pluggy-1.6.0
PyQt5 5.15.11 -- Qt runtime 5.15.17 -- Qt compiled 5.15.14
rootdir: /Users/paul/code/a/pandas
configfile: pyproject.toml
plugins: anyio-4.9.0, xdist-3.8.0, localserver-0.9.0.post0, cov-6.2.1, qt-4.5.0, hypothesis-6.135.32, cython-0.3.1
collected 21 items                                                                                                                                                                                                                                                                   

pandas/core/base.py .....................

------------------------------------------------------------------------------------------------------------ generated xml file: /Users/paul/code/a/pandas/test-data.xml -------------------------------------------------------------------------------------------------------------
================================================================================================================================ slowest 30 durations ================================================================================================================================

(30 durations < 0.005s hidden.  Use -vv to show these durations.)
================================================================================================================================= 21 passed in 0.03s =================================================================================================================================

[jorisvandenbossche]

Thanks for the PR! Looks good, just one small comment

[jorisvandenbossche]

I suppose this was removed accidentally? (and that also causes the failing CI)

[paulbehrisch]

Yes, let me fix that! Sorry about that.