Gnome terminal will not start

Probably the symlink /usr/bin/python3 points to python3.6, which it should not. Fix it by running these commands:

sudo rm /usr/bin/python3
sudo ln -s python3.5 /usr/bin/python3

You don't have to point Python3 to python3.5 , just running the commands:

cd /usr/lib/python3/dist-packages/gi/
sudo cp _gi.cpython-35m-x86_64-linux-gnu.so _gi.cpython-36m-x86_64-linux-gnu.so
sudo cp _gi_cairo.cpython-35m-x86_64-linux-gnu.so _gi_cairo.cpython-36m-x86_64-linux-gnu.so

Proper change of default python3 is done via:

sudo update-alternatives --config python3

Although copying the _gi_cairo.cpython-35m-x86_64-linux-gnu.so from python3.5 to python3.6 could resolve the issue, but would break your library consistency. And should be avoided.