Replace string concatenation with Python f-strings

[Scott3142]

The string concatenations throughout the material that look like

print(name + "is " + str(age) " years old.")

are left over from the Java course from which this is adapted and look really smelly in Python.

This issue is here to remind me to change them to f-strings at some point when I have time.

[Scott3142]

Note to self: This will break the testing procedure for Python 3.5 since f-strings were introduced in 3.6.

Will also need to update the testing routines and drop the 3.5 requirement if f-strings are to be used.