Option to use preinstalled Julia environment/Project.toml

PythonCall use pyjuliapkg. This is will install a new environment, even in mybinder where the julia dependencies are already precompiled, however in another julia Project.

It would be really helpful if we could easily use PythonCall with different setups of Julia dependencies. I.e. it would be great to turn of pyjulipkg and just use pre-exiting Julia project.

Start by tracing how PythonCall invokes pyjuliapkg to create and select Julia environments. Define an option that uses an existing Julia Project.toml instead of installing a new environment, then verify it works with precompiled dependencies such as those available in mybinder.

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