Python library is present (like on manylinux), where 0.17.2 returned None, and
previous versions returned a non-existent path. Note that adding ``REQUIRED``
to ``find_package(PythonLibs`` will fail, but it is incorrect (you must not
link to ``libPython.so``) and was really just injecting a non-existent path
before.
Bug fixes
---------
* Keep ``get_python_library`` return type string if python lib non-existing
for now in :pr:`959`.
* Avoid 'not found' warning if libs are not found by FindPythonExtensions in :pr:`960`.
* FindNumPy should not call FindPythonLibs in :pr:`958`.