Now that the migration to Python 3 is finished, we should not talk about
Python 2, which is not supported anymore.

- get rid of outdated py2-requirements.txt file.
- README: remove "PYTHON 3 MIGRATION" outdated section (it's now done)
  and update various other info.
- content/about/meta/index.md: update various info to Python 3 instead
  of v2.