azure-pipelines.yml: switch to Python 3.7
* switch to Python 3.7 (UsePythonVersion@0 task)
* use "pip install --user"
Fixes: https://pagure.io/freeipa/issue/8030
Signed-off-by: François Cami <fcami@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>