Update init so that sys.executable points to python

[jonatanklosko]

When running hf/transformers, during the first time a model is downloaded, I noticed a long message with BEAM executable usage. It turns out that there is a Python module that may spawn another Python OS process when certain shared resources are opened, such as shared memory, semaphores, to make sure the resources are never leaked (ref). The issue is that it tries to spawn sys.executable (effectively argv[0]), which points to the BEAM executable. This PR fixes it, such that it points to a Python executable.

This introduces a breaking change to Pythonx.init, and I decided to remove it from the API altogether, until there is an actual use case for it. In practice, I don't see a good reason no to use Pythonx.uv_init, especially that getting the paths correctly across OSes can be tricky.