Loosen dependencies, fix signature parsing regular expression

Hey there, I am using this project together with https://github.com/digitalbazaar/py-http-signature/pull/1 on a python3-based project and wanted to contribute some of the fixes I made along the way.

Regarding the regular expression - I'm not quite sure how the original was supposed to work but this explicitily matches the correct group (as I had problems with the regex matching more than just the signature).

Let me know if I can do anything else to help!