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/ceval_macros.h

@@ -62,8 +62,9 @@
 #ifdef Py_STATS
 #define INSTRUCTION_STATS(op) \
     do { \
+        PyStats *s = _PyStats_GET(); \
         OPCODE_EXE_INC(op); \
-        if (_Py_stats) _Py_stats->opcode_stats[lastopcode].pair_count[op]++; \
+        if (s) s->opcode_stats[lastopcode].pair_count[op]++; \
         lastopcode = op; \
     } while (0)
 #else