title: shutil.which() not finding programs without appending file extension
body: <p>Trying to use <code>shutil.which()</code> to determine if git is installed. From the docs, I see <code>which()</code> on Windows should use PATHEXT to know which file extensions to append when searching. However the following occurs when using the interpreter:</p>
  
  <pre><code>&gt;&gt;&gt; import os
  &gt;&gt;&gt; import shutil
  &gt;&gt;&gt; os.getenv('PATHEXT')
  '.COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;'
  &gt;&gt;&gt; shutil.which('git')
  &gt;&gt;&gt; shutil.which('git.exe')
  'C:\\Program Files\\Git\\cmd\\git.exe'
  </code></pre>
  
  <p>This is in a virtual environment created using VirtualEnv Wrapper in D:\Envs if that makes any difference.</p>