Fix the BlockingIOError on Python for Windows

Patching `proactor_events.py`

--- proactor_events.py	2018-01-23 13:55:46.665000000 +0700
+++ proactor_events_fixed.py	2018-01-23 12:22:08.319000000 +0700
@@ -493,7 +493,23 @@
             f.add_done_callback(self._loop_self_reading)
 
     def _write_to_self(self):
-        self._csock.send(b'\0')
+        # Original code
+        # self._csock.send(b'\0')
+
+        # This may be called from a different thread, possibly after
+        # _close_self_pipe() has been called or even while it is
+        # running.  Guard for self._csock being None or closed.  When
+        # a socket is closed, send() raises OSError (with errno set to
+        # EBADF, but let's not rely on the exact error code).
+        csock = self._csock
+        if csock is not None:
+            try:
+                csock.send(b'\0')
+            except OSError:
+                if self._debug:
+                    logger.debug("Fail to write a null byte into the "
+                                 "self-pipe socket",
+                                 exc_info=True)
 
     def _start_serving(self, protocol_factory, sock,
                        sslcontext=None, server=None, backlog=100):