Python 3.7 / PEP 479 compliance fix for iterator nestedly raising StopIteration

[csachs]

peekiter() raises StopIteration from within a nested generator, this behavior was soft deprecated in Python 3.6 and hard deprecated in Python 3.7. (I found out because some code broke ;))

Fixes the following DeprecationWarning in Python 3.6:

python test_png.py 
...................purepng/png/png.py:1480: DeprecationWarning: generator 'peekiter.<locals>.gen' raised StopIteration
  for i, row in enumrows:
.........................................................purepng/png/png.py:1318: DeprecationWarning: generator 'peekiter.<locals>.gen' raised StopIteration
  for row in inrows:
.......
----------------------------------------------------------------------
Ran 83 tests in 3.169s

OK

And a uncaught exception in Python 3.7…