Basing on blog post by Debugging of CPython processes with gdb
One more way to resolve the issue is to source python3.5-gdb.py file.
Run command info auto-load to check that files are loaded (if so)
and if not — load it manually:
source /usr/share/gdb/auto-load/usr/bin/python3.5-gdb.py
(! path may be different).
Then py-bt should work!
EDITED: changed py version