> what about open(.., encoding="fromcookie")?

Please don't do that. It remembers me the magical str.encode() method of 
Python2. I prefer simple API with limited features: if you want to open a 
Python script, use tokenize.open(), if you want to open an XML document use 
any XML library, etc. It remembers me also the discussion about detecting BOM 
in text files: issue #7651. There was no consensus, and so the issue is still 
open.

For this particular issue, I prefer a specific function tokenize.<choose a 
function name>.