Update changes.rst for release 1.7.3
Changes
=======
+Version 1.7.3
+-------------
+
+* Fixed SQLAlchemy 1.4 removed the Engine.contextual_connect method
+ By :user:`juarezr`, :issue:`545`.
+
+* How to use convert with custom function and reference row
+ By :user:`javidy`, :issue:`542`.
+
Version 1.7.2
-------------
