Update sklearn refs

Author: kbattocchi

econml/_ensemble/_ensemble.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code is a fork from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_base.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_base.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/_tree_exporter.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains some snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_export.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_export.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/grf/_base_grf.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_forest.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_forest.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/grf/_base_grftree.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_classes.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_classes.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/grf/_criterion.pyx

@@ -6,7 +6,7 @@
 # Licensed under the MIT License.
 #
 # This code contains some snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_criterion.pyx
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_criterion.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/policy/_forest/_forest.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/ensemble/_forest.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/ensemble/_forest.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/policy/_forest/_tree.py

@@ -2,7 +2,7 @@
 # Licensed under the MIT License.
 #
 # This code contains snippets of code from:
-# https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_classes.py
+# https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_classes.py
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/tree/_criterion.pyx

@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_criterion.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_criterion.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/tree/_splitter.pyx

@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_splitter.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_splitter.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #

econml/tree/_tree.pyx

@@ -5,7 +5,7 @@
 # Copyright (c) Microsoft Corporation. All rights reserved.
 # Licensed under the MIT License.
 #
-# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/tree/_tree.pyx
+# This code is a fork from: https://github.com/scikit-learn/scikit-learn/blob/main/sklearn/tree/_tree.pyx
 # published under the following license and copyright:
 # BSD 3-Clause License
 #