[rtags] Allow to 'derive' rtags Since some releases, rtags (structure underlying uicfg) have selector and may be copied using something like: new_rtags = deepcopy(original_rtags) new_rtags.__module__ = __name__ new_rtags.__select__ = custom_selector The problem is that starting from that, both rtags wil diverge and changes in original_rtags won't be considered, while we usually want to set a few specific rules only in new_rtags. To fix this problem, this cset introduces the notion of "derivated/parent" rtag, eg: new_rtags = original_rtags.derive(__name__, custom_selector) Beside easier copying, when using the above method changes in original_rtags which are not overriden by new_rtags will be considered since it only hold its specific rules but look among its parent chain for non-found keys. Along the way, flake8 unittest_rtags. Closes #16164880

[tox]
envlist = py27,py34,flake8

[testenv]
sitepackages = true
deps =
  pytest
commands =
  {envpython} -m pytest {posargs:test}

[testenv:flake8]
skip_install = true
whitelist_externals =
  flake8
deps =
  flake8
commands = flake8

[flake8]
exclude = cubicweb_%(cubename)s/migration/*,test/data/*,.tox/*