title: open KNOWNFOLDERID with askopenfilename in Tkinter / Python
body: <p>I want to set my initialdir to a KNOWNFOLDERID like Documents or Desktop.
  Is this even possible?
  I tried like this, but instead it open up the directory of my python project.</p>
  <pre><code>from tkinter import *
  from tkinter import filedialog
  
  root = Tk()
  
  root.filename = filedialog.askopenfilename(
      initialdir=&quot;%USERPROFILE%/Documents&quot;,
      title=&quot;get a file&quot;,
      filetypes=((&quot;.jpg files&quot;, &quot;*.jpg&quot;), (&quot;.png files&quot;, &quot;*.png&quot;)) 
  )
  
  root.mainloop()
  </code></pre>