LintMiddleware not Python 3 compatible

[NiklasMM]

Accessing a Werkzeug-Application with an active LintMiddleware raises an error:

AttributeError: 'ClosingIterator' object has no attribute 'next'

That's because this part of it is not Python 3 compatible:

werkzeug/src/werkzeug/middleware/lint.py

Lines 127 to 133 in a220671

	class GuardedIterator(object):
	def __init__(self, iterator, headers_set, chunks):
	self._iterator = iterator
	self._next = iter(iterator).next
	self.closed = False
	self.headers_set = headers_set
	self.chunks = chunks

A possible fix would be to set self._next to iter(iterator).__next__ if run under Python 3.