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..."
 

tests/travis/setup.sh

@@ -1,7 +1,19 @@
 #!/bin/bash
 
 if [ ${TASK} == "lint" ]; then
-    pip install --user  cpplint 'pylint==1.4.4' 'astroid==1.3.6' 
+    if [ ${TRAVIS_OS_NAME} == "osx" ]; then
+        wget -O conda.sh https://repo.continuum.io/miniconda/Miniconda3-latest-MacOSX-x86_64.sh
+    else
+        wget -O conda.sh https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh
+    fi
+    bash conda.sh -b -p $HOME/miniconda
+    export PATH="$HOME/miniconda/bin:$PATH"
+    hash -r
+    conda config --set always_yes yes --set changeps1 no
+    conda update -q conda
+    # Useful for debugging any issues with conda
+    conda info -a
+    conda create -n python3 python=3.7
 fi
 
 
@@ -18,6 +30,6 @@ if [ ${TASK} == "python_test" ] || [ ${TASK} == "python_lightweight_test" ] || [
     conda update -q conda
     # Useful for debugging any issues with conda
     conda info -a
-    conda create -n python3 python=3.5
+    conda create -n python3 python=3.7
     conda create -n python2 python=2.7
 fi