"Enhancements" (non-bugfix feature changes) can only be applied to future versions.  However, you are asking for the reversion of an intentional feature change made in a 'bugfix' release# for (I believe) 3.1.  Before the change, as I remember, truncating to 0 *did* move the file pointer back to 0.  As I remember, Guide von Rossum requested the change and Antoine Pitrou made it.  

https://docs.python.org/3/library/io.html#io.IOBase.seek
new says "The current stream position isn’t changed."

If you also want to change the stream position, do it with seek(), perhaps before the truncate.

# This change in a bugfix release, a violation the rule stated above, broke the code of multiple people.  (We thereafter strengthened the  policy.)  To fix my code, I had to add a seek(0).  I put it before truncate(0), so I know that this works.