Bump to release v0.5.1

NEWS

+0.5.1:
+    - Fix bug #14199, building docs with python3
+    
 0.5.0:
     - Updated to work with python3 or python2
     - Improve extension path loading semantics to follow XDG recommendations

configure.ac.in

@@ -13,7 +13,7 @@ dnl *** Version information ***
 dnl ***************************
 m4_define([txp_version_major], [0])
 m4_define([txp_version_minor], [5])
-m4_define([txp_version_micro], [0])
+m4_define([txp_version_micro], [1])
 m4_define([txp_version_tag], [])
 m4_define([txp_version_build], [])
 m4_define([txp_version], [txp_version_major().txp_version_minor().txp_version_micro()ifelse(txp_version_tag(), [], [], [txp_version_tag()-txp_version_build()])])