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

The HTML plugin is using the `subprocess` library to fire up the
web browser that will show the report.  In theory, we could be
using our own `avocado.utils.process` for that, but given the
comments in that section, I believe there may be caveats.

As a fix for the immediate issue at hand (pylint catching a failure on
a Python 3 environment about using `file`), let's just use the
definitions from the `subprocess` module itself.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/html/avocado_result_html/__init__.py

@@ -222,10 +222,9 @@ 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=inout,
-                         stdout=inout, stderr=inout,
+        subprocess.Popen(cmd, close_fds=True, stdin=subprocess.DEVNULL,
+                         stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
                          preexec_fn=setsid)
 
     def _render(self, result, output_path):