Prepare CHANGELOG for release 1.3.1

Author: molinav

CHANGELOG.md

@@ -5,7 +5,7 @@ is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and the
 project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 since version 1.3.0.
 
-## [Unreleased]
+## [1.3.1] - 2022-01-22
 
 ### Added
 - Support for Python 3.10 (issues [#530] and [#531]).
@@ -891,7 +891,9 @@ https://github.com/matplotlib/basemap/issues/228
 https://github.com/matplotlib/basemap/issues/179
 
 [Unreleased]:
-https://github.com/matplotlib/basemap/compare/v1.3.0...develop
+https://github.com/matplotlib/basemap/compare/v1.3.1...develop
+[1.3.1]:
+https://github.com/matplotlib/basemap/compare/v1.3.0...v1.3.1
 [1.3.0]:
 https://github.com/matplotlib/basemap/compare/v1.2.2rel...v1.3.0
 [1.2.2]: