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

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

@@ -1184,23 +1184,29 @@ def tostring(element, encoding=None, method=None):
 # @defreturn sequence
 # @since 1.3
 
-def tostringlist(element, encoding=None, method=None):
-    data = []
-    class DataStream(io.BufferedIOBase):
-        def writable(self):
-            return True
+class _ListDataStream(io.BufferedIOBase):
+    """ An auxiliary stream accumulating into a list reference
+    """
+    def __init__(self, lst):
+        self.lst = lst
+        
+    def writable(self):
+        return True
 
-        def seekable(self):
-            return True
+    def seekable(self):
+        return True
 
-        def write(self, b):
-            data.append(b)
+    def write(self, b):
+        self.lst.append(b)
 
-        def tell(self):
-            return len(data)
+    def tell(self):
+        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