Optimize tostringlist by taking the stream class outside the function. It's...

Optimize tostringlist by taking the stream class outside the function. It's now 2x faster on short calls. Related to #1767933

Lib/xml/etree/ElementTree.py

@@ -1184,9 +1184,12 @@ def tostring(element, encoding=None, method=None):
 # @defreturn sequence
 # @since 1.3
 
-def tostringlist(element, encoding=None, method=None):
-    data = []
-    class DataStream(io.BufferedIOBase):
+class _ListDataStream(io.BufferedIOBase):
+    """ An auxiliary stream accumulating into a list reference
+    """
+    def __init__(self, lst):
+        self.lst = lst
+        
     def writable(self):
         return True
 
@@ -1194,13 +1197,16 @@ def tostringlist(element, encoding=None, method=None):
         return True
 
     def write(self, b):
-            data.append(b)
+        self.lst.append(b)
 
     def tell(self):
-            return len(data)
+        return len(self.lst)
 
-    ElementTree(element).write(DataStream(), encoding, method=method)
-    return data
+def tostringlist(element, encoding=None, method=None):
+    lst = []
+    stream = _ListDataStream(lst)
+    ElementTree(element).write(stream, encoding, method=method)
+    return lst
 
 ##
 # Writes an element tree or element structure to sys.stdout.  This