ENH: Add statsmodels

[bashtage]

Add statsmodels which is already Python 3 only

If you're adding a project to the Python 3 statement, please ensure you have:

• Discussed dropping Python 2 support with all maintainers
• Discussed signing the statement with all maintainers
• Added a suitable logo if the project has one
  • Approximately square logos generally fit better than banners
  • Compressed the logo to save bandwidth (run scripts/squash-images.sh on it)
• Checked the Netlify preview made from the pull request

[takluyver]

Thanks!