[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
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