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/.jenkins.d/util.sh b/.jenkins.d/util.sh
index 8ddc4ba..8077a74 100644
--- a/.jenkins.d/util.sh
+++ b/.jenkins.d/util.sh
@@ -16,6 +16,7 @@
     set ${saved_xtrace}
     return ${ret}
 }
+export -f has
 
 sudo_preserve_env() {
     local saved_xtrace
@@ -35,3 +36,4 @@
     set ${saved_xtrace}
     sudo env "${vars[@]}" "$@"
 }
+export -f sudo_preserve_env