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This is needed as rqlcontroller is listed as deps in signedrequest.
This creates the following error in signedrequest:

cubicweb._exceptions.ConfigurationError: cycles in graph: rqlcontroller -> signedrequest

This is cubicweb related and not a cycle in deps directly.

because of CSRF

It is usefull for download through PyPi, because it avoids to
download the new version that only support Python 3 if it is still
a Python 2 project.

- Python 2 support removed.
- File cubicweb-rqlcontroller.spec seemed to be usefull only for
  Python 2.6, so we removed it.
- We use mainly Debian 10 "Buster" that ships python3.7 for
  python3, so we consider that we won't support previous versions
  anymore. This cube is maintained just a bit and users should
  upgrade, so it does not sound a problem to remove support for
  things that should be upgraded.
- With the same logic, the current last version of CubicWeb
  (3.27.3) is now required. In fact, the needed code in CubicWeb
  is not yet in a version, so it is not accurate, because the
  next one will be needed, but we would like not to wait to
  publish a new version of rqlcontroller.

We decide to support only the newest version of CubicWeb. Like
this, it is easier to maintain and encourages users to upgrade.

- move cube files to new cube folder
- update setup.py, MANIFEST.in, __pkginfo__.py

Drop useless stuff and make the file flake8-compliant.

Related to #17047898.

Related to #17047898.