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/sysmodule.c

@@ -2281,7 +2281,9 @@ static PyObject *
 sys__stats_on_impl(PyObject *module)
 /*[clinic end generated code: output=aca53eafcbb4d9fe input=43b5bfe145299e55]*/
 {
-    _Py_StatsOn();
+    if (_Py_StatsOn() < 0) {
+        return NULL;
+    }
     Py_RETURN_NONE;
 }