shebang should be specific to python version (PEP394)

preferred form is #!/usr/bin/env python3 as gnucash now only works with
python3

bindings/python/example_scripts/new_book_with_opening_balances.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 # new_book_with_opening_balances.py -- Replicate the account structure of a
 # book and apply basis opening balances from the original