I think there is a case to be made for adding "let" to Python. Would you like to write a PEP? I recommend that you find a co-author who can help you write a convincing motivation to sway the Steering Council (not me).
(Why do I propose "let" and not "const"? It seems "let" has become the industry standard language. The other day I saw a reference to a LET() function in Excel. :-)