thirdparty: vendor cbor2 python library CBOR stands for Concise Binary Object Representation, which is a data format which is very compact and extensible. This patch moves the python library which can serilaize and deserialize python objects to/from cbor formats. The library is taken from https://github.com/agronholm/cbor2/ from commit 84181540f6eb650437e3f73cd104a65661fe8e67. Unrequired files from the cbor library - docs/, tests/, setup.py, setup.cfg, and tox.ini - have not been vendored. There is another python library for cbor at https://github.com/brianolson/cbor_py/ which is used in evolve extension and was imported in initial version of this series. That library though contains C code and is bit faster, but has known bugs around serializing nested structures, is unmaintained, raises an Exception object instead of a more dedicated Error type. So, it's better to use a bug free and actively maintained library. This library is not yet used and will be used in later commits. # no-check-commit because we are importing a third library module Differential Revision: https://phab.mercurial-scm.org/D2750