optional_plugins.html: Use `open` to get file

"file" is not available on python3, let's use "open" to get file
instance.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

optional_plugins/html/avocado_result_html/__init__.py

@@ -222,7 +222,7 @@ class HTMLResult(Result):
         setsid = getattr(os, 'setsid', None)
         if not setsid:
             setsid = getattr(os, 'setpgrp', None)
-        inout = file(os.devnull, "r+")
+        inout = open(os.devnull, "r+")
         cmd = ['xdg-open', html_path]
         subprocess.Popen(cmd, close_fds=True, stdin=inout,
                          stdout=inout, stderr=inout,