Migrate pylint check to Python 3 (#4381)

* Migrate lint to Python 3

* Fix lint errors

* Use Miniconda3 to use Python 3.7

* Use latest pylint and astroid

tests/travis/run_test.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
 
 if [ ${TASK} == "lint" ]; then
+    source activate python3
+    conda install numpy scipy
+    python -m pip install cpplint pylint astroid
     make lint || exit -1
     echo "Check documentations..."