Consider using python-libzim instead of zimply-core

[libzim](https://github.com/openzim/libzim) is the official implementation of the ZIM specification. It has [Python bindings](https://github.com/openzim/python-libzim) and is more tested and maintained than ZIMply/ZIMply-core.

One issue though is that [no binaries are provided](https://github.com/openzim/python-libzim/issues/91) for libzim at the moment, so we have to compile it ourselves. I've not been successful so far and this was my prime reason for choosing ZIMply, but the latter has its issues: https://github.com/abdnh/anki-zim-reader/issues/3