[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

[test] Use bare unittest in unittest_rtags

[rtags] Docstring fix

[cleanup] flake8 rtags.py

[pkg] Require passlib >= 1.7.0 Since bc9d901cb9e6 we use 1.7.0 API, update packaging accordingly.

[web/http_headers] do not crash when IfModifiedSince is empty string (closes #16527954)