pyOpenSSL and service_identify should be in an extras_require

[jolynch]

Since fido doesn't actually import anything from pyOpenSSL and is meant to be installed as a library, we shouldn't be depending on it as an install_requires.

Instead we should use an extras_requires (see docs)

This way we don't have to depend on cryptography which is a pain to compile.

[jolynch]

https://github.com/twisted/twisted/blob/69b30c9a9b74f44180b8118ce9fde69a70bcfe15/src/twisted/python/_setup.py#L90

Twisted already has an extra_requires for precisely this reason :-D