title: Case insensitive argparse choices
body: <p>Is it possible to check <a href="https://docs.python.org/3/library/argparse.html#choices">argparse choices</a> in case-insensitive manner?</p>
  
  <pre><code>import argparse
  choices = ["win64", "win32"]
  parser = argparse.ArgumentParser()
  parser.add_argument("-p", choices=choices)
  print(parser.parse_args(["-p", "Win32"]))
  </code></pre>
  
  <p>results in:</p>
  
  <pre><code>usage: choices.py [-h] [-p {win64,win32}]
  choices.py: error: argument -p: invalid choice: 'Win32' (choose from 'win64','win32')
  </code></pre>