Use 'is'/'is not' instead of '==' to avoid Py3 warnings about compari…

…ng disparate types.

src/twisted/web/distrib.py

@@ -114,7 +114,7 @@ def __init__(self, request):
         self.request = request
 
     def finished(self, result):
-        if result != server.NOT_DONE_YET:
+        if result is not server.NOT_DONE_YET:
             assert isinstance(result, str), "return value not a string"
             self.request.write(result)
             self.request.finish()

src/twisted/web/server.py

@@ -311,7 +311,7 @@ def render(self, resrc):
                 body = epage.render(self)
         # end except UnsupportedMethod
 
-        if body == NOT_DONE_YET:
+        if body is NOT_DONE_YET:
             return
         if not isinstance(body, bytes):
             body = resource.ErrorPage(