Use python-3.9 for dependency scanning

Dependency scanning needs to be done with a newer version of python otherwise pip cannot newer package versions:

Could not find a version that satisfies the requirement

This can be fixed by adding the following to the .gitlab-ci.yml file:

# Use the python-3.9 version to run dependency scanning, see: https://gitlab.com/gitlab-org/gitlab/-/issues/344859
gemnasium-python-dependency_scanning:
  stage: test
  image: registry.gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python:2-python-3.9