Please give us
1. the exact Python version used. 3.2.3? or something earlier?
2. A minimal but complete example that we can run. What is 'headers'?
3. The complete traceback, not just the last two entries.
4. The result of running with the newer 3.3.0, if you possibly can. Perhaps the problem has already been fixed.

While line numbers have changed, even in 3.2.4 in repository, 3.2-3.4 all have

        request = '%s %s %s' % (method, url, self._http_vsn_str)
        # Non-ASCII characters should have been eliminated earlier
        self._output(request.encode('ascii'))

Since there is nothing earlier in the function that would eliminate non-ascii, there must be an assumption about what happens earlier in the call chain. That might have already been fixed, which is why we need an example to test.