[docs] fix RTD builds due to conda's problems (#2725)
[docs] fix RTD builds due to conda's problems (#2725)

* fix RTD builds

* more pins

* hotfix

* removed unnecessary pinning

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