make FSImage.__str__ available in Python 3 (#282)

* make FSImage.__str__ available in Python 3

In both Python 2 and 3, the tag() method returns the respective str type, so
the result can always be used for __str__. Without having this method
available in Python 3, the image tag cannot be included, for example, in
Chamaeleon page templates by simply accessing the image object, as that uses
i18n which in turn casts the object to the respective text (unicode) type. Not
implementing __str__ in Python 3 results in the object representation instead
of the tag in that case.

CHANGES.rst

@@ -36,6 +36,10 @@ Bugfixes
 - Accept bytes and text as cookie value.
   (`#263 <https://github.com/zopefoundation/Zope/pull/263>`_)
 
+- ``__str__`` of an Image object now returns the image HTML tag in
+  Python 3 as it already did on Python 2.
+  (`#282 <https://github.com/zopefoundation/Zope/pull/282>`_)
+
 
 4.0b4 (2018-04-23)
 ------------------

src/OFS/Image.py

@@ -876,9 +876,8 @@ def update_data(self, data, content_type=None, size=None):
     def __bytes__(self):
         return self.tag().encode('utf-8')
 
-    if PY2:
-        def __str__(self):
-            return self.tag()
+    def __str__(self):
+        return self.tag()
 
     security.declareProtected(View, 'tag')
     def tag(self, height=None, width=None, alt=None,

src/OFS/tests/testFileAndImage.py

@@ -27,6 +27,8 @@
 from zope.lifecycleevent.interfaces import IObjectModifiedEvent
 from zope.lifecycleevent.interfaces import IObjectCreatedEvent
 
+import six
+
 here = os.path.dirname(os.path.abspath(__file__))
 filedata = os.path.join(here, 'test.gif')
 
@@ -328,3 +330,8 @@ def test_interfaces(self):
         from OFS.interfaces import IWriteLock
 
         verifyClass(IWriteLock, Image)
+
+    def test_text_representation_is_tag(self):
+        self.assertEqual(six.text_type(self.file),
+                         '<img src="http://nohost/file"'
+                         ' alt="" title="" height="16" width="16" />')