- Versions of CubicWeb that are less than 3.26 are not supported
  anymore, because we don't support them.
- Setup that 2.7 is the minimum Python version.
- cubicweb-bootstrap.spec was used for Python 2.6, that is why
  it has been removed.
- Indicate that it intented to work in Python 2 and 3.
- Define a minimum version of `six`, the one of Debian 10 "Buster",
  because we don't test on older versions.
- HTTPS is better than HTTP.