When installing python3-qwt from Debian stretch (using apt-get install), python3-pyqt4 is also installed, even if python3-pyqt5 is available.
After testing python3-qwt with python3-pyqt5 and a dummy package of python3-pyqt4 (allowing to bypass the dependency on it), the impression is that python3-qwt is working well with python3-pyqt5.
The dependency on python3-pyqt4 should be made optional. If python3-pyqt5 is already installed, python3-pyqt4 should not be installed.
When installing python3-qwt from Debian stretch (using apt-get install), python3-pyqt4 is also installed, even if python3-pyqt5 is available.
After testing python3-qwt with python3-pyqt5 and a dummy package of python3-pyqt4 (allowing to bypass the dependency on it), the impression is that python3-qwt is working well with python3-pyqt5.
The dependency on python3-pyqt4 should be made optional. If python3-pyqt5 is already installed, python3-pyqt4 should not be installed.