set pymongo version to 3.8 (#1666)
set pymongo version to 3.8 (#1666)

there are some issues with 3.9, so fix the version while investigating
superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core