It seems that for whatever reason, running as a non-root user, you don't have access to read the pbkdf2.py python library...
Have you checked the permissions on both the directory (/usr/local/lib/python3.5/dist-packages/) and the file itself?
How did you originally install Python... and the packages? Did you use pip?
Right, so the version that I referred to in my original post was from the Debian Stretch backports, the most recent avaialble in the Debian world. I prefer to stick to the Debian way as using pip3 messes up the python environment with libraries etc that end up conflicting with apt-get.
That said, I also tried the pip3 way as described on the Electrum website with the most recent version. That too failed and was giving me the exact same problems. Yes, I think it is a permissions problem but I thought pip3 install would take care of that? I've just checked permissions and I think they a bit messed up on that folder. See below, does this look alright to you? BTW, my user is member of the staff group. You can see that most of the stuff in that folder was installed yesterday.
lun mag 28, 10:48 ][user@machine:~]ls -al /usr/local/lib/python3.5/dist-packages/
totale 132K
4,0K drwxrwsr-x 20 root staff 4,0K mag 27 04:20 .
4,0K drwxrwsr-x 3 root staff 4,0K ott 14 2017 ..
4,0K drwx--S--- 4 root staff 4,0K mag 27 03:22 dns
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 dnspython-1.15.0.dist-info
4,0K drwx--S--- 3 root staff 4,0K mag 27 03:22 ecdsa
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 ecdsa-0.13.dist-info
4,0K drwx--S--- 5 root staff 4,0K mag 27 03:22 electrum
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 Electrum-3.1.3.dist-info
4,0K drwx--S--- 3 root staff 4,0K mag 27 03:22 google
4,0K drwx--S--- 3 root staff 4,0K mag 27 03:22 jsonrpclib
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 jsonrpclib_pelix-0.3.1.dist-info
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 pbkdf2-1.3.dist-info
12K -rw------- 1 root staff 9,9K mag 27 03:22 pbkdf2.py
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 protobuf-3.5.2.post1.dist-info
4,0K -rw------- 1 root staff 539 mag 27 03:22 protobuf-3.5.2.post1-py3.5-nspkg.pth
4,0K drwx--S--- 3 root staff 4,0K mag 27 03:22 pyaes
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 pyaes-1.6.1.dist-info
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 __pycache__
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 PySocks-1.6.8.dist-info
4,0K drwx--S--- 5 root staff 4,0K mag 27 03:22 qrcode
4,0K drwx--S--- 2 root staff 4,0K mag 27 03:22 qrcode-6.0.dist-info
4,0K -rw------- 1 root staff 2,9K mag 27 03:22 sockshandler.py
32K -rwxr-xr-x 1 root staff 32K mag 27 03:22 socks.py
4,0K drwx--S--- 3 root staff 4,0K mag 27 03:22 usr