At the moment Tiled for macOS does not ship with a Python plugin. This is because I’m not familiar enough with macOS to know how this can be achieved. When Tiled was using Python 2 it was straight-forward because macOS ships with it.
I wonder if it would suffice to install Python 3 with Homebrew when building Tiled, to get the Python plugin to be included. And then to require installation of Python 3 by the user before this plugin can be enabled. One problem with this, as we have encountered on Windows, is that when the version of the Python 3 library does not match, Tiled may crash when you try to enable the plugin. :-/
In the end, the proper way to support Python may be to ship the Python framework along with it, but I don’t really want to go along that path.
Note that Tiled 1.3 is extendable using JavaScript. Would that be an option for you? It is much easier to provide cross-platform support for JavaScript than for Python.