Which version of pip are you using? I see you've been trying 3 different approaches: default (presumably Python 2.7?) pip, pip3 and the APT repository versions.
You could try updating the Python 3.x version of pip using pip itself, rather than the apt-installed version. This seems to work for me (on Ubuntu 20.04):
❯ pip3 --version
pip 20.0.2 from /usr/lib/python3/dist-packages/pip (python 3.8)
❯ pip3 install -U pip
Collecting pip
Using cached pip-20.3.3-py2.py3-none-any.whl (1.5 MB)
Installing collected packages: pip
Successfully installed pip-20.3.3
❯ pip --version
pip 20.3.3 from /home/omf/.local/lib/python3.8/site-packages/pip (python 3.8)
❯ pip3 --version
pip 20.3.3 from /home/omf/.local/lib/python3.8/site-packages/pip (python 3.8)
❯ pip3 install -U PyQT5
Defaulting to user installation because normal site-packages is not writeable
Requirement already satisfied: PyQT5 in /usr/lib/python3/dist-packages (5.14.1)
Collecting PyQT5
Using cached PyQt5-5.15.2-5.15.2-cp35.cp36.cp37.cp38.cp39-abi3-manylinux2014_x86_64.whl (68.3 MB)
Requirement already satisfied: PyQt5-sip<13,>=12.8 in /home/omf/.local/lib/python3.8/site-packages (from PyQT5) (12.8.1)
Installing collected packages: PyQT5
Successfully installed PyQT5-5.15.2
However, Mnemosyne wouldn't launch with this version of PyQT5 since this Ubuntu APT repo only has QT 5.12.
I got past that with this:
❯ pip3 install -U PyQT5==5.14
Then it still wouldn't launch because:
>> ImportError: cannot import name 'QtWebEngineWidgets' from 'PyQt5' (/home/omf/.local/lib/python3.8/site-packages/PyQt5/__init__.py)
It seemed I already had the required package installed via apt, and it was needed for two other programs. I temporarily uninstalled them with:
❯ sudo apt remove python3-pyqt5.qtwebengine
❯ pip3 install PyQTWebEngine==5.14
Then I reinstalled the programs that depended on python3-pyqt5.qtwebengine, and Mnemosyne works again.
Python packages can be a bit of a pain. That's why I prefer to deal with them in "userspace" rather than system-wide "site-packages". But even still, every few months I'll run into problems and have to reinstall things. Well, it makes things exciting at least.
Oisín