Mac zip files cause an error

Apple in their wisdom creates metadata files like `__MACOSX/._worksheet.sws` in zip files.  The problem is that Sage tries to open such files as `sws` files and predictably fails.  This then leads to all sorts of unpleasantness.

I'm not sure a regex is the best way to handle it, but I couldn't find out how to use the zip metadata to ignore it based on some flag, but maybe someone else knows better.