Fix check for dmenu (open file mode, dependency on which).

See <https://github.com/proxypoke/quickswitch-for-i3/pull/24>.

--- quickswitch-i3-2.2/quickswitch.py	2016-09-18 19:59:10.257765542 +0200
+++ quickswitch-i3-2.2/quickswitch.py	2016-09-21 10:43:01.277577819 +0200
@@ -24,9 +24,10 @@
 
 import argparse
 import subprocess
-import os
 import re
 
+from distutils.spawn import find_executable
+
 try:
     import i3
 except ImportError:
@@ -37,11 +38,8 @@
 
 def check_dmenu():
     '''Check if dmenu is available.'''
-    devnull = open(os.devnull)
-    retcode = subprocess.call(["which", "dmenu"],
-                              stdout=devnull,
-                              stderr=devnull)
-    return True if retcode == 0 else False
+    executable = find_executable("dmenu")
+    return executable != None
 
 
 def dmenu(options, dmenu):