gh-131253: free-threaded build support for pystats (gh-137189)

Allow the --enable-pystats build option to be used with free-threading.  The
stats are now stored on a per-interpreter basis, rather than process global.
For free-threaded builds, the stats structure is allocated per-thread and
then periodically merged into the per-interpreter stats structure (on thread
exit or when the reporting function is called). Most of the pystats related
code has be moved into the file Python/pystats.c.

Python/initconfig.c

@@ -2810,12 +2810,6 @@ _PyConfig_Write(const PyConfig *config, _PyRuntimeState *runtime)
         return _PyStatus_NO_MEMORY();
     }
 
-#ifdef Py_STATS
-    if (config->_pystats) {
-        _Py_StatsOn();
-    }
-#endif
-
     return _PyStatus_OK();
 }