Independent branch structure for py-heptapod

Similar to #1673 (closed), without the upstream factor: in the last couple of releases, the version of py-heptapod was the same in all branches. As it is fixed in python/common.req, this is not a problem. Stable branch shifts and other branch management procedures are just merging empty changesets, so we should

close existing stable branches
reopen stable branches only when needed
have HDK use a branch head (tag not being convenient in this case), overriddable in gdk.yml

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