I'd like to update the Portal system properties using the Python API. The documentation seems to indicate that it can be done. However, making an assignment to System.properties doesn't appear to udpate the system properties.
from arcgis.gis import GIS
gis = GIS(gis_base_url, user, password)
system_properties = gis.admin.system.properties
system_properties['privatePortalURL'] = system_properties['WebContextURL']
gis.admin.system.properties = system_properties