Move chained assignment detection to cython for Python 3.14 compat

[jorisvandenbossche]

Draft exploring a solution for #61368

[jbrockmendel]

Some commented-out code needs to be removed, otherwise looks nice

[jbrockmendel]

Can you merge main to see if that fixes the ci

[jorisvandenbossche]

Only remaining failure is the free-threaded build, where a bunch of tests fail with a missing warning, because the check in cython does not seem to work for free-threaded python (https://github.com/pandas-dev/pandas/actions/runs/18678375696/job/53253518246?pr=62070).

To move forward with this PR, will include "3.14+ and freethreaded" condition to the CHAINED_WARNING_DISABLED constant (i.e. disable the warning check for now in such env), and commented on #61368 to see if we can solve this properly.

[jbrockmendel]

Is this ready?

[jbrockmendel]

@mroeschke any idea if this is ready?

[jbrockmendel]

Discussed at today's dev call. This is working except for the free-threading build. We're going to merge this and follow up with a fix for the freethreading build when a solution presents itself.

[jbrockmendel]

thanks @jorisvandenbossche