urllib request mocking for pystac/load_stac related tests?

The openEO python client uses the popular requests module for HTTP requests. Testing of that is currently done with the requests_mock module.

For STAC support (e.g. load_stac) we are depending more and more on pystac which uses urllib/urllib3 internally. This is obviously not covered by requests_mock so we need something else as urllib mock utility. I might have missed something obvious but didn't find anything up to date yet

Because of lack of real URL mocking for urrlib, we currently workaround that with local paths to temp files, which is accepted by pystac. e.g.:
https://github.com/Open-EO/openeo-python-client/blob/026881df787de007205e110f765260aeaf2e9876/tests/rest/test_connection.py#L2950-L2957

That works for simple use cases, but it hinders testings of real HTTP functionality (e.g. handling of failed requests, handling relative URLs, ...)

Start with tests/rest/test_connection.py around lines 2950-2957 and review how temporary local paths currently stand in for HTTP resources. Investigate an up-to-date way to mock urllib/urllib3 requests used by pystac, then cover failed requests and relative URLs so real HTTP behavior can be tested without local files.