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):
'submit' value='switch'/></form></td></tr>
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<div class='path'>path: <a href='/guix/log/?id=8b6b097714962beeb675cad6425b2417dc564dab'>root</a>/<a href='/guix/log/gnu?id=8b6b097714962beeb675cad6425b2417dc564dab'>gnu</a>/<a href='/guix/log/gnu/packages?id=8b6b097714962beeb675cad6425b2417dc564dab'>packages</a>/<a href='/guix/log/gnu/packages/acl.scm?id=8b6b097714962beeb675cad6425b2417dc564dab'>acl.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/packages/acl.scm?id=8b6b097714962beeb675cad6425b2417dc564dab&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-06-17 14:38:32 +0200'>2023-06-17</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=ff690faf66d8f37ee6b679fcb7cf37e369624ee3'>gnu: Use target-hurd?, system-hurd? instead of hurd-target?, hurd-system?.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/hurd.scm (hurd-target?): Remove.
(hurd-system?): Move to...
* guix/utils.scm (system-hurd?): ...here.
* gnu/packages/*: Update all users, removing (gnu packages hurd) include where
now unused.
</span></span></td><td>Janneke Nieuwenhuizen</td></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-10-18 14:37:26 +0000'>2021-10-18</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=e486b2b674badc80627b11077b7df2ac1cab92d8'>Merge remote-tracking branch 'signed/master' into core-updates</a></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2021-10-18 12:25:47 +0200'>2021-10-18</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=9cda21cf20a5c9bdf97e3a6d6c8f901fc3e4307d'>gnu: python-pylibacl: Add missing brace in description.</a><span class='msg-avail'>...<span class='msg-tooltip'>Regression introduced in 133f780039e63798535e4f52a4d9f1810ea5b2ab.

* gnu/packages/acl.scm (python-pylibacl)[description]: Add missing brace.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-10-15 17:30:56 +0200'>2021-10-15</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=133f780039e63798535e4f52a4d9f1810ea5b2ab'>gnu: python-pylibacl: Improve description.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/acl.scm (python-pylibacl)[synopsis]: Use @acronym{}.
[description]: Turn into a full sentence, mark up, and edit for GNU.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-10-15 17:30:56 +0200'>2021-10-15</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=a9b8443cc530fbf404f86c215ed77b8baeb0d108'>gnu: python-pylibacl: Do not return #t from custom phases.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/acl.scm (python-pylibacl)[arguments]: Don't explicitly
return #t from phases.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2021-07-23 17:26:45 +0200'>2021-07-23</span></td><td><a href='/guix/commit/gnu/packages/acl.scm?id=5d435b2235d5e269499be2ea67613c164e65efb6'>gnu: acl: Update to 2.3.1.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/acl.scm (acl): Update to 2.3.1.
</span></span></td><td>Marius Bakke</td></tr>