users' guide: don't use f strings in the python script to ensure compatibility with python 3.5

This should fix the debian 9 nightly build which ships with python 3.5.

Even though the testsuite driver requires python 3.7+, the python installed in the container is not added to $PATH and thus the script called by sphinx to build the users' guide is fails since the python version on debian 9 is 3.5, f strings were released as part of python 3.6.