I have added comments for tests and documentation. The implementation looks correct. But I prefer simpler solution on this stage. Here is something between zipfile-open-w5.patch and zipfile-open-w8.patch, close to zipfile-open-w5.patch with fixed issues.

* Writer doesn't use look and seeking.
* Writing while there are open read handlers is permitted.
* Reading while there is open write handler is forbidden.

This works in all cases when current code works. The ability to read while there is open write handler is separate new feature (and I don't know whether there is a need in it).