Since you can not easily install Steem-Python on a stock Ubuntu 16.40 anymore, I switched to Ubuntu 17.10, which comes with the required Python 3.6 or higher.

The installation was not as smooth as I had wished for.

This is what it says in the repository:

---

Installation

You can install steem-python with pip:

pip install -U steem

Source: https://github.com/steemit/steem-python

---

On a fresh Ubuntu 17.10 this will not work at all.
Anyways, you want to actually do this:

pip3 install -U steem

But for that to work, you need the following:

sudo apt install python3-pip
pip3 install setuptools
pip3 install scrypt
pip3 install wheel
pip3 install pytest
sudo apt-get install libssl-dev

Now you can install Steem-Python:

pip3 install steem

I hope I could save someone some time, because figuring this out took me longer than I would like to admit.