obfsproxy should use C implementation of UniformDH

We are currently using a C implementation of UniformDH that is quite slow (even with gmpy2 for mod exp).

Yawning implemented UniformDH in C using OpenSSL and we should use his library.

He posted an obfsproxy patch in #11015 (closed) : https://trac.torproject.org/projects/tor/attachment/ticket/11015/0001-Add-support-for-using-py-uniformdh.patch

And the implementation can be found in: https://github.com/Yawning/py-uniformdh