build+ci: switch to python3

Also in this commit:
 * Sync CI scripts with other projects
 * Modernize docs/conf.py
 * Replace redmine_issue extension with sphinx.ext.extlinks
 * Minor updates to docs/doxygen.conf.in
 * Cleanup README.md and INSTALL.rst

Refs: #5095
Change-Id: I614e876e6aed73659f4f4cea01813f4604599263
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
 #