Check usage of dateutil

[m-mohr]

We use dateutil twice in the code.

It seems to work, but it's a pypi dependency which we have not listed in the qpip dependency list.

https://pypi.org/project/python-dateutil/

[m-mohr]

It's a dependency of the openEO Python client, so it works for now. We'd need to list it once the Python client doesn't use it anymore.