Client usage example didn't work with python3

Good point from Priler that our example should use BytesIO rather than StringIO to work under python3... https://trac.torproject.org/projects/tor/ticket/16502

Client usage example didn't work with python3
28cbc64f · Damian Johnson · 842da03c · 28cbc64f
Commit 28cbc64f authored Jul 4, 2015 by Damian Johnson
--- a/docs/_static/example/client_usage_using_pycurl.py
+++ b/docs/_static/example/client_usage_using_pycurl.py
+import io
 import pycurl
-import StringIO

 import stem.process

@@ -13,7 +13,7 @@ def query(url):
  Uses pycurl to fetch a site using the proxy on the SOCKS_PORT.
  """

-  output = StringIO.StringIO()
+  output = io.BytesIO()

  query = pycurl.Curl()
  query.setopt(pycurl.URL, url)