[ci][docs] Restrict Sphinx version (#4267)
[ci][docs] Restrict Sphinx version (#4267)


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