This patch changes to logic to:

   if env.var. set and non-empty:
       if env.var. is an integer:
           set flag to that integer
   if flag is zero: # [actually, <= 0 --GvR]
       set flag to 1

   Under this patch, anyone currently using
   PYTHONVERBOSE=yes will get the same output as before.

   PYTHONVERBNOSE=2 will generate more verbosity than
   before.

   The only unusual case that the following three are
   still all equivalent:
   PYTHONVERBOSE=yespleas
   PYTHONVERBOSE=1
   PYTHONVERBOSE=0