Redirect orca process stdout to devnull to prevent deadlock (plotly#1262)

This only seemed to be causing a problem on windows after
more than 20 images had been exported.

The reason stdout had previously been set to subprocess.PIPE
was to prevent orca's stdout from being displayed in python
shell. Setting it to devnull accomplishes the same thing
but avoid the deadlock problem

Author: jonmmease

Diff for: plotly/io/_orca.py

@@ -1086,10 +1086,6 @@ def shutdown_server():
                     # Kill parent process
                     orca_state['proc'].terminate()
 
-                    # Retrieve standard out and standard error to avoid
-                    # warnings
-                    output, err = orca_state['proc'].communicate()
-
                     # Wait for the process to shutdown
                     child_status = orca_state['proc'].wait()
                 except:
@@ -1174,8 +1170,9 @@ def ensure_server():
 
             # Create subprocess that launches the orca server on the
             # specified port.
+            DEVNULL = open(os.devnull, 'wb')
             orca_state['proc'] = subprocess.Popen(cmd_list,
-                                                  stdout=subprocess.PIPE)
+                                                  stdout=DEVNULL)
 
             # Update orca.status so the user has an accurate view
             # of the state of the orca server