Drop a left-over 3.22 deprecation warning

[doc] fix reference to ~src/cubes in tutorial ~src/cubes refer to the $HOME/cubes directory of "src" user while ~/src/cubes refer to $HOME/src/cubes directory of current user.

[doc] replace cubicweb-ctl start by cubicweb-ctl pyramid Since twisted support is gone.

[doc] make the tutorial use a python3 virtualenv

[doc] Update README "Getting started" * install using pip and with pyramid in a virtualenv * link to pyramid.ini documentation * Use pyramid to start application since twisted support is gone

Flake8 cubicweb/devtools/httptest.py

Remove Twisted web server Twisted web server is not used anymore and has been superseded by pyramid many years ago. Furthermore, our usage is not compatible with Python 3. So we drop the "etwist" sub-package. As a consequence, "all-in-one" configuration type gets dropped as it was Twisted-specific. We resurrect it in cubicweb/pyramid/config.py by only keeping options used by the "pyramid". Similarly, we introduce a AllInOneCreateHandler in cubicweb/pyramid/pyramidctl.py that is basically the one that lived in cubicweb/etwist/twctl.py and is used to create the "all-in-one" instance. Added a TODO here about "pyramid.ini" that could be generated at the end of bootstrap() method. In cubicweb/devtools/httptest.py, CubicWebServerTC is now equivalent to CubicWebWsgiTC and the latter is dropped.

Separate twisted-specific configuration from WebConfigurationBase class This is in preparation of dropping the Twisted web server backend. We thus draw a clearer line between what's specific to twisted from what's generic by moving WebConfigurationBase class into cubicweb/web/webconfig.py and only keeping twisted-related in AllInOneConfiguration (still living in cubicweb/etwist/twconfig.py).