* [VERSION] Enhance version.py to support git-describe.
This PR enhances version.py with a --git-descrbe option
which allows it to generate a git describe based version
tag for potential dev related nightly packaging during the
development cycle.
The behavior of the normal relase remains the same.
Note that the version.py still modifies the files inplace
and we only recommend using it during a clean clone based workflow.
The setup.py is also updated to take advantage of the version.
Note that the git info is already captured by the c++ side in a previous PR.
The tool is mainly used to create PEP compatible python wheels.
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Tianqi Chen
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