calibre: Pin zeroconf dependency to 0.31
calibre: Pin zeroconf dependency to 0.31

This prevents https://bugs.launchpad.net/calibre/+bug/1936889 which has
been patched [1] in calibre master but not yet released.

[1]: https://github.com/kovidgoyal/calibre/commit/4f9e83e6426483b6cc0929c61f9207c33c573fec

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