Docs: Type annotation reference issues

[soxofaan]

current sphinx build has these warnings:

WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.datacube.DataCube.execute_local_udf": name 'xarray' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.vectorcube.VectorCube": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.mlmodel.MlModel": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.mlmodel.MlModel.load_ml_model": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.job.BatchJob": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.job.RESTJob": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.conversions.datacube_from_file": name 'XarrayDataCube' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.conversions.datacube_plot": name 'XarrayDataCube' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.conversions.datacube_to_file": name 'XarrayDataCube' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.rest.udp.RESTUserDefinedProcess": name 'Connection' is not defined
WARNING: Cannot resolve forward reference in type annotations of "openeo.udf.xarraydatacube.XarrayDataCube.plot": name 'matplotlib' is not defined

[soxofaan]

This might help: https://github.com/tox-dev/sphinx-autodoc-typehints#dealing-with-circular-imports