Investigate polars warning regarding python fork()

[larsevj]

When running ert with polars 14, you get the following warning:

/Users/LEVJE/.local/share/uv/python/cpython-3.12.7-macos-aarch64-none/lib/python3.12/asyncio/subprocess.py:224: RuntimeWarning: Using fork() can cause Polars to deadlock in the child process.
In addition, using fork() with Python in general is a recipe for mysterious
deadlocks and crashes.

The most likely reason you are seeing this error is because you are using the
multiprocessing module on Linux, which uses fork() by default. This will be
fixed in Python 3.14. Until then, you want to use the "spawn" context instead.

See https://docs.pola.rs/user-guide/misc/multiprocessing/ for details.

  transport, protocol = await loop.subprocess_exec(

Should investigate if this is at all relevant for ert, and if not silence the warning.

[berland]

This is probably related to usages of fork() that is not immediately followed by an exec(). If all our forks stem from launching shell subprocesses, this warning is not relevant to Ert. Some background info: https://lisper.in/fork-exec-python