Merge (common parts of) PythonCall and PyCall?

I realize PyCall is legacy (at least to me), but it will still be used (some wrappers for Python code depend in it) for the foreseeable future.

It also has one pro, faster startup, so I was thinking does it make sense to merge some parts is some sort of PyCallBase that likely ordinary users wouldn't use.

If you use PythonCall and PyCall together you have to be careful, but that package would make sure to download the same Python for you (or use the system one, maybe?), at least making sure both will work together.

No files or tests are named. Start by comparing the shared responsibilities of PythonCall and PyCall, especially Python download or system-Python selection and their coexistence constraints. Define the scope and compatibility criteria for a possible common base before considering implementation.

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