gh-113360: Fix doctest failure when __test__ is set to falsy value

[sobolevn]

Refs #112109

Thanks a lot for the detailed analysis!

• Issue: pytest --doctest-modules crashes on file-level __test__ = False #113360

[serhiy-storchaka]

Heh. I wrote exactly same changes. But now I think that perhaps fixing the documentation is more correct, since this feature did not work since 2.4.

[serhiy-storchaka]

It was fixed in other way, by changing the outdated documentation.