Initial build and test changes

Two small initial changes:

1. Don't test on NEO for Python 3.

   NEO is still not ported to Python 3, so Python 3 tests should not use this backend.

2. Fix wendelin_cy_searh_in_dirs

   The interface of the function search_include_directories has changed in recent versions of Cython. This updates the replacement used by wendelin so that it works for both newer and older versions.

   Note that wendelin-core still does not work with Cython >= 3, as that version refuses to compile Python functions that can throw C++ exceptions (apparently, mixing C++ exceptions and Cython-generated code is not considered safe).