ZipPath patch so that plugins works with non-writable plugins cache

New submission from grossetti <None>:

As described in the email thread [http://twistedmatrix.com/pipermail/twisted-python/2009-February/019244.html Twisted plugins and py2exe], I had problems using plugins with py2exe when the files are compressed in a Zip archive. A potential solution was to patch plugin.py to not try to update the cache file (or not creating it at all), btu that wasn't seen as a good solution. Another solution was to have py2exe create the cache and copy the cache files into the Zip archive, this is the path I took. While implementing this, I ran into a small problem, the `open()` funtion's signature is not the same in `twisted.python.filepath.FilePath` as it is in `twisted.python.zippath.ZipPath` and so I got exceptions. The patch I am submitting make's `ZipPath`'s open function take an optional `mode` para
 m and makes sure only read-only modes are passed. Also, if there could be a page in the wiki concerning py2exe it would be great, it could explain the code to make py2exe & Twisted w/ plugins behave like expected. I also joined a py2exe `setup.py` example on how to do that. This ticket is in some ways related to tickets #996 & #2409

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Type     : enhancement
Component: core
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/3826