windows7-64-py2.7-msi discovers the Twisted version by importing Twisted. The version is required to build an MSI. The discovery code fails if anything other than the version is written out when importing twisted to learn the version, though, as seen on a recent build:
This doesn't show up in trunk due to luck, thanks to nothing in twisted/__init__.py currently triggering any warnings. The above build is from a branch which adds a zope.interface import to twisted/__init__.py.