Python's StringIO doesn't do well with `with` statements

I need to stub tempfile and StringIO seemed perfect. Only that all this fails in an omission:

In [1]: from StringIO import StringIO
In [2]: with StringIO("foo") as f: f.read()

--> AttributeError: StringIO instance has no attribute '__exit__'

What's the usual way to provide canned info instead of reading files with nondeterministic content?