Can I sign an X509 certificate entirely in Python?

I'm working within the Google App Engine (GAE) python 2.7 runtime.

I need sign (and potentially also generate) an X509 certificate (.csr)

I found several guides, that rely on PyOpenSSL.

As I understand it, PyOpenSSL is is wrapper around OpenSSL and not available in App Engine.
Pycrypto is available, but does not have a clear pre-built signing method for X509.

How can I sign an X509 .csr using only python?