Make `SyntaxWarning` for invalid escape sequences better reflect their intended deprecation

[Rosuav (Chris Angelico)]

@umarbutler

I think double blackslash is preferred because it is guaranteed to work. If someone runs message = 'Hello\n World and\or fizz buzz' and you suggest they do message = r'Hello\n World and\or fizz buzz', you’ve now broken the \n.

… but I would suggest they do message = "Hello\n World and/or fizz buzz" because it’s more grammatically correct.