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):
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<tr><td><span title='2017-09-22 00:05:08 +0200'>2017-09-22</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=fbc31dc1247d3a494246e69f3cf28476af9eb9d6'>services: Move 'session-environment-service-type' to pam.scm.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-05-08 23:11:01 +0200'>2017-05-08</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=e586257b550918fefaab3970f2c314d6285f54ab'>system: Allow root to run "su" without password.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2016-11-06 19:56:43 +0100'>2016-11-06</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=290ad2243c0ce2919918f64fec5652b5a60466a8'>gnu: Remove comment which is factually incorrect.</a><span class='msg-avail'>...</span></td><td>John Darrington</td></tr>
<tr><td><span title='2016-07-19 23:50:03 +0200'>2016-07-19</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=909147e43f8c9f8c9b9d33597d5dd83facca699c'>services: Add pam-limits-service.</a><span class='msg-avail'>...</span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2016-02-06 12:01:42 +0100'>2016-02-06</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=12c00bca92e3eef2b86565924bbefc39397b5497'>system: pam: 'pam-root-service-type' can be extended with transformations.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-02-06 12:01:42 +0100'>2016-02-06</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=d7bce31c36469167226df9bed95be39db595c2c8'>system: pam: Export accessors.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2015-12-05 10:51:49 +0800'>2015-12-05</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=af9908ff56fc9263c5f343c23002ac11300e195e'>system: pam: Honor /etc/environment.</a><span class='msg-avail'>...</span></td><td>宋文武</td></tr>
<tr><td><span title='2015-11-03 18:11:45 +0100'>2015-11-03</span></td><td><a href='/guix/commit/gnu/system/pam.scm?id=6e82863463c641571e852291481e2b64401c2fe2'>system: Rename (gnu system linux) to (gnu system pam).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>