[issue38435] Start the deprecation cycle for subprocess preexec_fn

[Gregory P. Smith]

Gregory P. Smith <greg at krypto.org> added the comment:

Doing the code inspection of existing preexec_fn= users within our codebase at work is revealing.  But those seem to be the bulk of uses.

I expect this deprecation to take a while.  Ex: if we mark it as PendingDeprecationWarning in 3.10, I'd still wait until _at least_ 3.13 to remove it.

Code using it often has a long legacy and may be written to run on a wide variety of Python versions.  It's painful to do so when features you need in order to stop using it are still only in modern versions.

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components: +Library (Lib)

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