build+ci: switch to python3

Also in this commit:
 * Sync CI scripts with other projects
 * Modernize docs/conf.py
 * Cleanup README.md
 * Use the official markdown version of the GPLv3

Refs: #5095
Change-Id: Ie607fcde1f6e41fa5efbbb41b21cb08583017442
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
 #