Some simple function in plpython3 is working?

Untrusted Python is little bit dangerous - Disconnection means so PostgreSQL crashed probably when it calls Python's validator. Untrusted Python allows to use any library (without limits), so there can be problems in some Python library initialization maybe.

You should to use some post mortem debugger to get call stack and investigate what is wrong.

https://wiki.postgresql.org/wiki/Getting_a_stack_trace_of_a_running_PostgreSQL_backend_on_Linux/BSD