Signature email requires ASCII on Python 2 but requires Unicode only "str" on Python 3

Signature_init in signature.c currently uses the s format string for parsing the email argument. This is inconsistent between Python 2 and 3. On Python 2, it accepts either text (unicode) or bytes (str) and implicitly encodes text using the default system encoding, which is ascii by default (and it seems like changing it is kind of weird and not really recommended by most people). Python 3, it only accepts text (str), and it's implicitly encodes it using UTF-8.

The name argument for Signature_init just uses O and accepts either text or bytes. It appears to implicitly encode text using UTF-8 and leaves bytes as-is. I would expect the email argument to work the same way.

So there's currently no way to pass in bytes on Python 3 and no way to pass in non-ASCII text on Python 2.

Start in signature.c at Signature_init and inspect how the email argument is parsed compared with the name argument. Verify behavior on Python 2 and Python 3; done means email accepts bytes on Python 3 and non-ASCII text on Python 2 consistently with name.