Avoid warning about unsealed object with GDAL main (3.9)

[jorisvandenbossche]

We get the warning "RuntimeWarning: OGRFeatureDefn::GetTemporaryUnsealer() called on a unsealed object" for various tests in the CI build using GDAL main (see eg https://github.com/geopandas/pyogrio/actions/runs/7870620687/job/21472183960)

Related to https://gdal.org/development/rfc/rfc97_feature_and_fielddefn_sealing.html

Didn't yet look in detail, but I assume this will require some code updates to fix our code and avoid the warning.