Python extension modules are usually expected to only export a single symbol, decorated with the PyMODINIT_FUNC macro and providing the module entry point. On versions of python >= 3.9, the python headers contain GNU symbol visibility attributes to mark the init function with default visibility; it is then safe to set the [[shared_module]] inherited kwarg gnu_symbol_visibility: 'hidden'.
In the interest of doing the right thing out of the box, this is now the default for extension modules for found installations that are new enough to have this set, which is not expected to break anything, but remains possible to set explicitly (in which case that will take precedence).