#18584: make doctest examples in email documentation pass.

With the exception of the 'as_string' call in the policy docs.
That one is a separate issue.

Note that when building the docs sphinx is complaining about
.. testcleanup:: being an invalid directive.  I don't know
why this is, as I'm following the sphinx docs...but fortunately
the action is to omit the text in the body, so the generated
documentation is correct.
diff --git a/Doc/library/email.iterators.rst b/Doc/library/email.iterators.rst
index d1f1797..6c7200f 100644
--- a/Doc/library/email.iterators.rst
+++ b/Doc/library/email.iterators.rst
@@ -33,14 +33,22 @@
    Thus, by default :func:`typed_subpart_iterator` returns each subpart that has a
    MIME type of :mimetype:`text/\*`.
 
+
 The following function has been added as a useful debugging tool.  It should
 *not* be considered part of the supported public interface for the package.
 
-
 .. function:: _structure(msg, fp=None, level=0, include_default=False)
 
    Prints an indented representation of the content types of the message object
-   structure.  For example::
+   structure.  For example:
+
+   .. testsetup::
+
+      >>> import email
+      >>> from email.iterators import _structure
+      >>> somefile = open('Lib/test/test_email/data/msg_02.txt')
+
+   .. doctest::
 
       >>> msg = email.message_from_file(somefile)
       >>> _structure(msg)
@@ -60,6 +68,10 @@
                   text/plain
           text/plain
 
+   .. testcleanup::
+
+      >>> somefile.close()
+
    Optional *fp* is a file-like object to print the output to.  It must be
    suitable for Python's :func:`print` function.  *level* is used internally.
    *include_default*, if true, prints the default type as well.