Remove structuralholes.py from needs_(num|sci)py

[Peiffap]

While reading the PR that recently updated structuralholes.py to understand #8087, I saw it added that file to the needs_numpy and needs_scipy lists in conftest.py. This excludes them from being tested when scipy (or numpy) is not available (strictly, this isn't the case, as the nodes argument is also considered in the if—but afaik there's no good reason to exclude these from the tests). However, the functions with a scipy implementation explicitly wrap import scipy in a try ... except using scipy where possible. As a result, the vanilla implementation does not get tested properly: when running the default tests (with scipy), the scipy implementation gets tested, and when running default-without-scipy, the file gets skipped.