Revert "HTML plugin: use subprocess.DEVNULL instead of custom open file"

The subprocess.DEVNULL is not available on Python2.

This reverts commit e51acbfa.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

optional_plugins/html/avocado_result_html/__init__.py

@@ -222,9 +222,10 @@ class HTMLResult(Result):
         setsid = getattr(os, 'setsid', None)
         if not setsid:
             setsid = getattr(os, 'setpgrp', None)
+        inout = file(os.devnull, "r+")
         cmd = ['xdg-open', html_path]
-        subprocess.Popen(cmd, close_fds=True, stdin=subprocess.DEVNULL,
-                         stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+        subprocess.Popen(cmd, close_fds=True, stdin=inout,
+                         stdout=inout, stderr=inout,
                          preexec_fn=setsid)
 
     def _render(self, result, output_path):