tomli 2.4.0 and TOML 1.1 - how to implement in pip?

[notatallshaw]

tomli 2.4.0 supports reading TOML 1.1, with no option to error on TOML 1.1 specific syntax, Python 3.15 is likely to do the same.

The question is what should pip do? I see there as being three choices that make sense:

• Vendor tomli 2.4.0 for Python < 3.15, so TOML 1.1 specific syntax is consistently accepted on all Python versions for pip
• Expand tomli 2.3.0 vendoring for all versions of Python, so TOML 1.1 specific syntax is consistently rejected on all Python versions for pip
• Keep the status quo, tomli 2.3.0 is vendored for Python < 3.11, so Python 3.15+ will accept TOML 1.1 specific syntax but Python < 3.15 will reject TOML 1.1 specific syntax

A more general packaging discussion is happening here: https://discuss.python.org/t/adopting-toml-1-1/105624

This issue is if any further pip-specific discussion is needed (unclear to me right now, I'm going to focus on the DPO thread before further considering pip-specific needs).

[notatallshaw]

I opened #13750 to vendor tomli 2.4.0 for Python < 3.15 but there was opposition, with one of the primary reasons being the cost of vendoring tomli for another ~4 years. I closed the PR until there is consensuses, further discussions are still happening on the DPO thread, and pip specific choices can be discussed here (instead of polluting a PR review).