...OK, got it (the answer to my question just below)
So I've a proposition : all bugs should be version rel_3_6, all ENH and
bugs attached to ENH should be version rel_3_8
version "master" being dedicated to ENH that still haven't made their
way to rel_3_8.
So you could find all bugs that needs backporting : all patches attached
to rel_3_6 !