fix nevow build prob for py2exe

fix nevow build prob for py2exe

nevow attempts to use pkg_resources to find the formless css file upon
import, if pkg_resources is available.  unfortunately using pkg_resources
to find files is not supported if the files are being loaded from a zip
archive (i.e. only source and egg), and further py2exe uses a zip bundle
for all the code and dependent libraries.  hence having both pkg_resources
and nevow built into an exe causes nevow to explode upon import.

this tells py2exe not to link pkg_resources into the target, so that
this behaviour isn't stimulated.  the side effect being that pkg_resources
isn't available.