requirements: bump python requirement to 3.9

[tmorrell]

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Description

Bump the python version check to 3.9, per https://inveniordm.docs.cern.ch/install/requirements/

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[utnapischtim]

@tmorrell as far as i know the lowest python version which invenio supports is version 3.7. i hope most of us are on python >= 3.9 but i think the bump should then only be to 3.7 or?