[doc] Update python3statement URL (#10179)

Author: tqa236

NEWS.md

@@ -2101,7 +2101,7 @@ This release marks a major milestone for the XGBoost project.
 ## v0.90 (2019.05.18)
 
 ### XGBoost Python package drops Python 2.x (#4379, #4381)
-Python 2.x is reaching its end-of-life at the end of this year. [Many scientific Python packages are now moving to drop Python 2.x](https://python3statement.org/).
+Python 2.x is reaching its end-of-life at the end of this year. [Many scientific Python packages are now moving to drop Python 2.x](https://python3statement.github.io/).
 
 ### XGBoost4J-Spark now requires Spark 2.4.x (#4377)
 * Spark 2.3 is reaching its end-of-life soon. See discussion at #4389.