Moving from python2 to python3

[Ken Brown]

This is a followup to a thread with subject "[PATCH] doc: Various fixes to 
makedocbook for python3.8" that began on the newlib list and then moved to

   https://cygwin.com/pipermail/cygwin-apps/2020-August/040442.html,

where it evolved into a discussion of moving from python2 to python3.  A recent 
post on the cygwin list reminded me that we never finished that discussion.  We 
left it with the question of what to do about scripts with a shebang that names 
python but might not work with python3.

With python2 reaching EOL in a month, it seems that we ought to finish that 
discussion and make the move.

Ken