package/python-flask: fix runtime python modules dependencies

- add python setuptools runtime dependency, fixes:
  ModuleNotFoundError: No module named 'pkg_resources'

- add python libxml2 runtime dependency, fixes:
  ModuleNotFoundError: No module named 'xml'

- add python ssl runtime dependency, fixes:
  AttributeError: 'NoneType' object has no attribute 'SSLContext'

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>