As @Bruce_Osborne said there is a possibility to turn on basic auth in the api/developer settings.
You could also run this command in the Karaf console:
bundle:stop org.openhab.core.io.rest.auth
which will restore the way everything was in OH2. Unfortunately you have to do this every time you restart openhab.
Also for everyone running small python scripts:
I’ve created HABApp which allows running python3 script as rules and abstracts the whole communication to openhab so you don’t have to deal with it.
You should definitely check it you as it’ll probably save you lots of headaches in the long run 