Rewrite runtime path lookup to libzim on macOS

[rgaudin]

Using the 0.0.3 release, I realized we have a packaging problem on macOS: the wrapper is looking for the libzim in standard location, which is good for those who have the libzim installed but not the expected behavior (that's why we bundle it!)

So this PR fixes that by telling the wrapper to look for the libzim next to it. The $ORIGIN rpath was already set for linux but macOS behaves differently (equivalent is @loader_path/)

Changes:

• Change --rpath argument based on platform
• Fix --rpath behavior on macOS python#36353
• Manually set the rpath on macOS (fix above sends proper command but output lib doesn't include it anyway)
• Changed version to 0.0.3.post0 so we can publish on PyPi

This has been tested on macOS and Linux via test.pypi.org