generate_units_documentation.py broken when used by renovate bot

see for instance !5504 (comment 2757318999):

Command failed: tools/generate_units_documentation.py
Traceback (most recent call last):
  File "/builds/sylva-projects/sylva-elements/renovate/renovate/repos/gitlab/sylva-projects/sylva-core/tools/generate_units_documentation.py", line 12, in <module>
    from packaging.version import Version
ModuleNotFoundError: No module named 'packaging'

This is because we lack the packaging python module in the image used by renovate bot.