Remove requests dependency

Requests was used in the OCSP requests because of a "bug" in certvalidator, it doesn't send a host header when connecting to an OCSP server. It is not a requirement according to the RFC's but it looks like quite a few servers require it, and it seems browsers do this because it works fine for those servers in browsers. So a pull request was created for certvalidator to send a header with the request. After this is merged (hopefully), we can remove requests and a lot of extra lines of code around it.