Renamed old lessons and added a few more
Script to change the names of files...

I named the exercises as "Lesson XX.py"...but the spaces proved to be a
problem (as usual)
So, this little code finds files that start with 'L' in a given
directory and updates them to 'exYYYYY' where 'YYYYY' are the last 5
chars from the original file name. In this case it was 'AA.py' where AA
was the lesson number