LightGBM

[docs] fixed building at Python 2.x (#997)

* fixed docs building at Python 2.x

* updated docs building guide

* get back Python 3 at Travis

* test commit for OSX

* be more precise with python version inpylint task

* turned warnings into errors

* removed linkchecker validation from osx

* removed warning about not included into any toctree

* docs config cleanup

* refined deprecation warnings mechanism

* refine docs mock mechanism

* be more precise with python version in check-docs task

* redused the number of code lines

* refined venv deployment

* rollback python version in check-docs task

* revert 'refine docs mock mechanism'. autodoc_mock_imports seems to be not working

* added targets for big images

* rollback to default python version in check-docs task

* break long lines for mobile view

* replaced pip with conda where it's possible