Thanks for looking at this!
I did it once again the whole procedure now, and now everything is working. Not sure what was different back then (may be I did sudo python3 setup.py build_ext -i ).
Any way, for record, this is how I did it:
==========================================
#then I updated PATH and PYTHONPATH to point to right places
# and now tests:
python3 setup.py build_ext -i
make test
Result is:
=========== 1681 passed, 24 skipped, 2 xfailed
===========================================
I am using Ubuntu 20.04.1 LTS with Python 3.8.2