There's a difference between python 3.6 and 3.7 that is used by electrum (I think). Debian (when I last used it) only had 3.6 on its repo meaning that the installs of electrum (from the website using git clone and curl - both methods) failed to load due to incompatibility issues.
Just tested it with Debian stable (testing has newer versions), and it seems it really only has python 3.5.3 in its repositories while electrum needs python 3.6.
However, it can be installed this way (python 3.6.3; just tested on debian stable):
wget https://www.python.org/ftp/python/3.6.3/Python-3.6.3.tgz
tar xvf Python-3.6.3.tgz
cd Python-3.6.3
./configure --enable-optimizations
make -j8
sudo make altinstall
You will have
python3 (3.5.3) and python3.6 installed.
Then you'll need to install pyqt5 via:
sudo pip3.6 install pyqt5
Afterwards electrum can be run using:
inside of the electrum directory.
Of course this also works with newer versions of python.