make-release.py script
ref #2391
This is a new `make-release.py` script which automates creation of the 'release PR' branch.
It has partial unittest coverage (large around version parsing/sorting/serializing) and always runs unittests prior to doing actual work. Most of the testing was done manually by using the ``--repo`` arg on a test repo, then reseting its state each time I needed a new test (to get around git checks). There is no other 'dry run' functionality.
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