Updating to Font Awesome 6

[jshipley]

Other than replacing the Font Awesome 5 icons with Font Awesome 6 icons, I did a couple of things

• Updated the lxml dependency so this will also work on newer versions of Python (fixes issue #5 and #7)
• Updated the README.md and demo.py screenshot
• Added support for aliases, so programs using the Font Awesome 5 version will still work
  • This means that fa-basketball, fa-basketball-ball, and basketball will all get the basketball icon
• Removed duplicate svgs directory, and removed the Font Awesome directories from svgs
  • This would mean that the Font Awesome icons would need to be downloaded again before running consolidate.py, but the most likely case for this would be a Font Awesome update anyway, and this prevents massive diffs when updating the icons
• Made consolidate.py write to tkfontawesome/svgs.py instead of tkfontawesome/svgs/svgs.py