Fix memory safety race condition in CPython 3.13 free-threaded mode

Add per-object locking via Py_BEGIN_CRITICAL_SECTION at all public-facing
entry points in the C extension to prevent use-after-free crashes when
iterating and mutating a MultiDict concurrently.

The root cause is _md_resize() reallocating md->keys without any
synchronization, causing concurrent iterators to access freed memory.
This fix wraps all public methods in Py_BEGIN_CRITICAL_SECTION(self),
matching CPython's own dict locking model. On GIL builds the macros
are no-ops (via pythoncapi_compat.h).

Adds tests/test_free_threading.py with a concurrent stress test that
previously crashed with SIGSEGV and now completes cleanly.

Author: rodrigobnogueira

CHANGES/1317.bugfix.rst

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+Fixed a memory-safety race condition resulting in segmentation faults (Use-After-Free) when iterating and modifying a ``MultiDict`` concurrently in CPython free-threaded mode (3.13t+). Read/Write accesses to the internal ``keys`` buffer are now wrapped in ``Py_BEGIN_CRITICAL_SECTION``.