Add free-threading support for Python 3.13+

Implements thread-safe concurrent database operations for Python's
free-threading mode using native locks where available.

- Uses native RW locks (pthread_rwlock_t on POSIX, SRWLOCK on Windows)
  to allow multiple concurrent readers
- Uses GIL on platforms that don't support these native locks
- Zero overhead when GIL is enabled (macros compile to no-ops)

Author: oschwald

HISTORY.rst

@@ -9,6 +9,10 @@ History
 * Databases can now be loaded from buffers. This can be done by passing in a
   buffer as the database and using mode ``MODE_FD``. Pull request by Emanuel
   Seemann. GitHub #234.
+* The C extension now supports Python 3.13+ free-threading mode and is
+  thread-safe for concurrent reads on platforms with pthread support (such as
+  Linux and macOS) and Windows. On other platforms, the extension will use
+  GIL-based protection.
 
 2.8.2 (2025-07-25)
 ++++++++++++++++++