apt-get/pacman installed stuff such as PyQt5 are nice to detect
(instead of installing it via pip inside the venv), however
it is probably better to isolate the venv from the ~global pip.
- set -e, and don't call deactivate (not needed; and with -e
if ./run_electrum errors it wouldn't run anyway)
- re PYTHONPATH
- I think the sane thing is to give priority to the virtualenv,
and only use system-packages as a fallback
- added more paths; tested that it now works for modern Ubuntu
and Manjaro
- use "python3 -m venv" instead of "virtualenv"
(as former is always(?) available now)
