Cover .format string method

[eah13]

the .format method is replacing the % operator, which will eventually be deprecated. Python 3 supports both, so we should cover both in Chapter 6.

I'll try to add this in the next few weeks if I can - teaching a summer course.

[fmaussion]

I think this should now even be replaced by f-string formatting ;-)

I just told my students not to learn about this section at all. Same here, I'll see if I find the time to rewrite this