fix(pacman): make the entire rule py2-compatible

One reference to subprocess.DEVNULL remained.

Signed-off-by: Pablo Santiago Blum de Aguiar <scorphus@gmail.com>

thefuck/rules/pacman.py

@@ -17,7 +17,7 @@ def __get_pkgfile(command):
     try:
         return subprocess.check_output(
             ['pkgfile', '-b', '-v', command.script.split(" ")[0]],
-            universal_newlines=True, stderr=subprocess.DEVNULL
+            universal_newlines=True, stderr=DEVNULL
         ).split()
     except subprocess.CalledProcessError:
         return None