Add support for Python 3.14

[czardoz]

Summary of changes

Had to jump through some hoops (hand-held by Claude & Codex).

generate_bindings.py + v3_14_0.rs:

• Suppressed unnecessary_transmutes warnings because bindgen generates conservative transmute calls that Rust 1.91+ flags.

python_interpreters.rs:

• Added 3.14 FrameObject impl: Mask tag bits from _PyStackRef.bits because Python 3.14 uses tagged pointers instead of raw pointers for f_executable and local variables.
  • Ref: gh-117139: Set up tagged pointers in the evaluation stack
  • Ref: gh-124218: Reference count contention with nested functions
• Fixed line number calculation (line 361): Changed >= to > because algorithm needs to process the entry where bytecode_address == lasti to get the correct line for that offset (was returning previous range's line number, causing intermittent test failures in Python 3.11-3.14).

python_data_access.rs:

• Refactored dict iteration: Python 3.14 changed inline values layout (stored after tp_basicsize instead of after PyObject) and uses PyDictKeyEntryHash for dk_kind==0 instead of legacy format.
  • Ref: gh-115789: Remove ma_version_tag from PyDictObject
  • Ref: gh-116032: Add PyDictKeyEntryHash

python_threading.rs:

• Added fallback to sysdict for thread name lookup because Python 3.14 can fail to import threading module into main sys.modules in some cases.

stack_trace.rs:

• Use F::read_local_address() trait method because Python 3.14 local variables are _PyStackRef (tagged pointers requiring masking).
  • Ref: gh-117139: Set up tagged pointers in the evaluation stack

tests/subprocesses.py:

• Force fork context because Python 3.14+ defaults to forkserver which creates extra intermediate process, breaking test expectations.
  • Ref: gh-84559: Change default multiprocessing start method to forkserver on Linux

tests/integration_test.rs:

• Added error message to test_recursive assertion for debugging line number issues.

[czardoz]

@benfred - any chance you can have a look next year? Thanks! 🙂