There are two problems:
- GPG missing key error on the
http://dl.bintray.com repository
EOFError: marshal data too short when Python3 tries to import urllib to launch software-properties-gtk (the "Software & Updates" settings window)
To resolve the GPG error, please follow @GAD3R's answer or any answer to How do I fix the GPG error "NO_PUBKEY"?
You decided to remove this repository from your software sources, which is also ok if you don't need it any more.
To resolve the Python EOFError, we need to eliminate some *.pyc files. Those are compiled byte code of a script, which gets dynamically created when a Python script runs to speed up future launches. They are faulty and don't match the original script any more for whatever reason. We can simply delete them as they are not really needed and will be recreated on the next launch.
We don't want a massacre but a small and effective removal of the faulty files only. Therefore we look at the error traceback from the software-properties-gtk output and find out that this error always happens when Python 3.4 tries to import urllib.parse. To avoid follow-up errors we simply remove all *.pyc files related to Python 3.4's urllib module:
sudo rm -r /usr/lib/python3.4/urllib/__pycache__