[doc] Use python3 for doc generation.

scripts/doc/append-xml-tags.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # This script augments the parameter documentation pages by information
 # such as if they are required/optional, their data typ and in which

scripts/doc/check-project-params.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # This script actually generates the QA page.
 # For its usage see generate-project-file-doc-qa.sh

scripts/doc/linked-xml-file.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # This script traverses the DATADIR (where the ctest input data is) and creates
 # out of every prj file found a file in a subdirectory of DOCAUXDIR.

scripts/doc/normalize-param-cache.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # This script takes the output of get-project-params.sh on stdin
 # and transforms it into a tabular representation for further

scripts/doc/print23.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 
 # Print statement that behaves the same for python 2 and 3.
 # E,g, print_(1.0, 2, "5") will always print the string "1.0 2 5".