build+ci: switch to python3

Also in this commit:
 * Sync CI scripts with other projects
 * Modernize docs/conf.py
 * Fix typo in docs/doxygen.conf.in
 * Cleanup README.md

Refs: #5095
Change-Id: If95b6f00e4f48a102054b47e14e9c021da069e71
diff --git a/waf b/waf
index 7ceee16..a1c5c96 100755
--- a/waf
+++ b/waf
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # encoding: latin-1
 # Thomas Nagy, 2005-2018
 #