Fix StringIO import for the example in usage.rst

For some reason people fixed the `print` call but not the import when #234 was brought up.

I also mentioned in in #369

Closes: #370
Pull-request: #370
Pull-request-sha: 8560bfe

Change-Id: I236eef05f5b45420035c522287fa66eb3cb21a00

doc/build/usage.rst

@@ -53,7 +53,7 @@ render with it, using the :meth:`~.Template.render_context` method:
 
     from mako.template import Template
     from mako.runtime import Context
-    from StringIO import StringIO
+    from io import StringIO
 
     mytemplate = Template("hello, ${name}!")
     buf = StringIO()
@@ -517,4 +517,3 @@ API Reference
 .. autofunction:: mako.exceptions.html_error_template
 
 .. autofunction:: mako.exceptions.text_error_template
-