Testing: Remove utf-8 declaration from add_header script; #6538

UTF-8 is the default source encoding in Python 3.
Declaring it is unncessary, unless Python 2 is also supported.
See also:
- https://peps.python.org/pep-3120/
- https://docs.python.org/3/howto/unicode.html#python-s-unicode-support
- https://stackoverflow.com/questions/14083111/should-i-use-encoding-declaration-in-python-3

tools/add_header

@@ -306,8 +306,7 @@ class PythonHeaderTemplate(HeaderTemplate):
         shebag_string = PythonHeaderTemplate._get_file_shebag(self.file_path) or ""
 
         return shebag_string \
-            + """# -*- coding: utf-8 -*-
-# Copyright European Organization for Nuclear Research (CERN) since 2012
+            + """# Copyright European Organization for Nuclear Research (CERN) since 2012
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.