Fixing `ImportError: No module named _ssl' while building Python from source in Ubuntu

This is just a reminder in case I ever come across this issue again.

Upon switching my Laptop's OS back to Ubuntu 16.04 today, I re-installed Python 3.6. Then I tried to pip install a module, and I found that the library ssl had a problem. The solution was, quite simply, to install libssl-dev (apparently this doesn't ship with Ubuntu), then rebuild Python 3.6.