Refactor code and use the Python 3.6's f-strings for the messages in byexample

[eldipa]

Describe the feature you'd like
Now the byexample dropped support for Python 3.5 we can improve the code using the f-strings that appeared in Python 3.6

For example:

var1 = 12
msg1 = "Message using old C-like style: %s" % var1
msg2 = "Message using old format-like style: {}". format(var1)

Things like that could be written as

msg3 = f"Message using the new f-string style {var1}"

Additional context (optional)
A lot of things will be modified, we need to have small commits and run the tests on each step to make sure that we are not making any mistake.

[eldipa]

Take a look to flynt, a tool that automates this.