It appears that Nevow is broken here: it's trying to import twisted.python
during the initial run of its setup script.  You can work around this by
reversing the order of the requirements.  (Currently, pkg_resources resolves
requirements in *reverse* input order.)

However, please note that Nevow is definitely broken; it should not be getting
its version information by importing itself.  Packages with dependencies should
*never* import from those dependencies during setup.py execution, whether
they're using setuptools or distutils.

At this point, I think it's established that the original problem (inability to
build+install in same process) is fixed, so I'm marking this resolved.