title: get list of named loglevels
body: <p>In my application, I'm using <code>python.logging</code> for logging.</p>
  
  <p>Now I want to control the loglevel interactively, so i created a combobox hat lets the user select "<em>ERROR</em>", "<em>WARN</em>", "<em>INFO</em>",...</p>
  
  <p>What I don't really like is that currently the values in the combobox are hardcoded.
  Instead,Ii would like to have a list of all "named" loglevels (e.g. both the system defaults, but also those added via <code>logging.addLevelName</code>; but not the fake generated loglevels like "<em>Level 42</em>")</p>
  
  <p>The best I have come up with so far is to use the <code>logging._levelNames</code> dictionary.</p>
  
  <p>But then this seems to be a private member, and I somehow have a bad feeling accessing it directly.</p>
  
  <p>So my question is: what's the proper way to list all currently defined "named" loglevels in Python.</p>