Bugfix: C extension not loaded for local tests on MacOS

[jannikmi]

When running pytest locally on MacOS the tests fail due to the C extension not being loaded (even though the project has been installed).

====================================================================================================== short test summary info ======================================================================================================
FAILED tests/utils_test.py::test_clang_extension_loaded - AssertionError: the clang extension not loaded, 
FAILED tests/utils_test.py::test_inside_polygon[test_case0-pt_in_poly_clang] - ValueError: Trying to use the clang implementation of the point in polygon algorithm while the C extension in not loaded.
FAILED tests/utils_test.py::test_inside_polygon[test_case1-pt_in_poly_clang] - ValueError: Trying to use the clang implementation of the point in polygon algorithm while the C extension in not loaded.
FAILED tests/utils_test.py::test_inside_polygon[test_case2-pt_in_poly_clang] - ValueError: Trying to use the clang implementation of the point in polygon algorithm while the C extension in not loaded.
================================================================================================== 4 failed, 106 passed in 10.05s ===================================================================================================

Note that these tests are running fine in the CI/CD pipeline in GitHub Actions (ubuntu)