[python] enforce a floor of Python 3.6 (fixes #5004) (#5006)
[python] enforce a floor of Python 3.6 (fixes #5004) (#5006)


microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM