gh-121103: Put free-threaded libraries in `lib/python3.14t`

[colesbury]

On POSIX systems, excluding macOS framework installs, the lib directory for the free-threaded build now includes a "t" suffix to avoid conflicts with a co-located default build installation.

• Issue: gh-121103

[colesbury]

pip, uv, venv, setuptools, seem to work, but I don't entirely know what I'm doing, so I'd appreciate a thorough review.

[hroncok]

Does this mean that all of the pure Python standard library files would need to be installed in lib/python3.14t as well? I.e. when I have both the "normal" and the freethreaded Python installed, do I need all the standard library modules twice?

[encukou]

I don't have very deep knowledge in this area, so I can't easily do a thorough review :( I'd trust @FFY00 and @hroncok's reviews if they can contribute their time for it.

[erlend-aasland]

Does this mean that all of the pure Python standard library files would need to be installed in lib/python3.14t as well? I.e. when I have both the "normal" and the freethreaded Python installed, do I need all the standard library modules twice?

As far as I can tell, from a quick look at this PR: yes.

[hroncok]

As far as I can tell, from a quick look at this PR: yes.

I thought so.

The real question is: Is this the outcome we anticipated?

If not, I suggest keeping the stdlib and platstdlib values intact.

[colesbury]

Yes, that was the intention. The goal is to separate the installation. We run into a lot of potential problems if we have partially overlapping stdlib files.

[erlend-aasland]

Yes, that was the intention. The goal is to separate the installation. We run into a lot of potential problems if we have partially overlapping stdlib files.

FTR, I totally agree with this.

[colesbury]

To expand on my previous comment: if we have kept a common stdlib and platstdlib, the Python files would be shared, but the .so files would not. So I'd be concerned that if someone installed, e.g. 3.13.0 default and 3.13.8 free-threaded to the same prefix, then the .py files and native .so libraries might not match. Keeping things separate in lib/python3.13t vs. lib/python3.13 avoids this problem.

[colesbury]

I plan to merge this tomorrow if there are no objections

[miss-islington-app[bot]]

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13. 🐍🍒⛏🤖

[bedevere-app[bot]]

GH-121631 is a backport of this pull request to the 3.13 branch.