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.

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,