Windows registry keys for Python file associations may have broken permissions, preventing installation

[davidsarah]

A build or install may fail on some Windows systems with the following error:

Setting up environment to run scripts for the current user...
error: Setup script exited with error: I was going to associate the
filetype .pyscript with Python.File for the current user, but that
won't work because the Python.File class does not exist in the
registry, as far as I can tell.
WindowsError(5, 'Access is denied')

See ​this tahoe-dev thread. The Windows system in that case was Windows 7 ultimate 64-bit.

This problem may occur because the Python.File key is not inheriting permissions from its parent (at least, clearing this permission inheritance is necessary to reproduce the problem on my machine).

For other reasons as well (#1302), we should probably stop relying on Python.File to point to the Python interpreter.