asyncio: Add Transport.is_closing()

See https://github.com/python/asyncio/pull/291 for details.
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index bf3b084..f75e89f 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -364,6 +364,9 @@
     def resume_reading(self):
         self._loop.add_reader(self._fileno, self._read_ready)
 
+    def is_closing(self):
+        return self._closing
+
     def close(self):
         if not self._closing:
             self._close(None)
@@ -548,6 +551,9 @@
             self._loop.remove_reader(self._fileno)
             self._loop.call_soon(self._call_connection_lost, None)
 
+    def is_closing(self):
+        return self._closing
+
     def close(self):
         if self._pipe is not None and not self._closing:
             # write_eof is all what we needed to close the write pipe