update version to 0.7.3

Author: niftynei

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [0.7.3rc3] - 2019-10-18: "Bitcoin's Proof of Stake"
+## [0.7.3] - 2019-10-18: "Bitcoin's Proof of Stake"
 
 This release named by @trueptolemy.