Create a test to ensure GeoAlchemy2 can be imported when Shapely is not installed

[adrien-berchet]

This change fixes the runtime NameError when Shapely isn't installed, but there’s no test ensuring import geoalchemy2 (or import geoalchemy2.shape) works in an environment without Shapely. Consider adding a regression test that imports/reloads geoalchemy2.shape with Shapely unavailable (e.g., via an import hook or sys.modules patch) so this doesn’t regress again.

Originally posted by @Copilot in #584