gh-63161: Fix tokenize detect_encoding() for non-ASCII coding

[vstinner]

• Issue: Non-UTF8 encoding line #63161

[serhiy-storchaka]

Please add a test for the coded cookie on the second line (and non-ascii first line).

Also add a test with specified ASCII encoding, but non-ASCII content that can still be decoded as UTF-8. E.g. '#coding=ascii €'.encode('utf-8') and corresponding for two lines.

[vstinner]

@serhiy-storchaka: I added more tests, please review the updated PR. Is it what you wanted?

[serhiy-storchaka]

Thank you for update. In two-line cases please use non-ASCII data in the first line, before the codec cookie. Test that the tokenizer uses correct encoding to decode comments in first lines.

It may be already tested elsewhere, but I would also add tests for non-ASCII data in the first and in the second comment lines, when no codec cookie is present (so UTF-8 should be used). For valid and invalid UTF-8.

I expect that the tokenizer correctly decodes files that match the explicit or implicit encoding, and reject files that do not match. And the interpreter should work the same.

[vstinner]

Ok, I added more tests. Please review the updated PR.