Prevent creating new console on Windows

When running Jupyter via pythonw e.g. pythonw -m qtconsole, jupyter_client launches new kernel via python.exe which is a console application on Windows - a side-effect of that is a new empty console window created and shown as long as kernel is running.

This patch adds CREATE_NO_WINDOW 0x08000000 to Windows specific creationflags. This flag is not exported by subprocess module therefore has to be provides numerically.

jupyter_client/launcher.py

@@ -103,8 +103,9 @@ def launch_kernel(cmd, stdin=None, stdout=None, stderr=None, env=None,
                 DUPLICATE_SAME_ACCESS, CREATE_NEW_PROCESS_GROUP
         # Launch the kernel process
         if independent:
-            kwargs['creationflags'] = CREATE_NEW_PROCESS_GROUP
+            kwargs['creationflags'] = CREATE_NEW_PROCESS_GROUP | 0x08000000 # CREATE_NO_WINDOW
         else:
+            kwargs['creationflags'] = 0x08000000 # CREATE_NO_WINDOW
             pid = GetCurrentProcess()
             handle = DuplicateHandle(pid, pid, pid, 0,
                                      True, # Inheritable by new processes.