gh-127266: avoid data races when updating type slots (gh-131174)

In the free-threaded build, avoid data races caused by updating type slots
or type flags after the type was initially created.  For those (typically
rare) cases, use the stop-the-world mechanism.  Remove the use of atomics
when reading or writing type flags.  The use of atomics is not sufficient to
avoid races (since flags are sometimes read without a lock and without
atomics) and are no longer required.

Lib/test/test_opcache.py

@@ -576,6 +576,7 @@ class TestRacesDoNotCrash(TestBase):
     # Careful with these. Bigger numbers have a higher chance of catching bugs,
     # but you can also burn through a *ton* of type/dict/function versions:
     ITEMS = 1000
+    SMALL_ITEMS = 100
     LOOPS = 4
     WRITERS = 2
 
@@ -619,7 +620,7 @@ class TestRacesDoNotCrash(TestBase):
                 __getitem__ = lambda self, item: None
 
             items = []
-            for _ in range(self.ITEMS):
+            for _ in range(self.SMALL_ITEMS):
                 item = C()
                 items.append(item)
             return items
@@ -790,7 +791,7 @@ class TestRacesDoNotCrash(TestBase):
                 __getattribute__ = lambda self, name: None
 
             items = []
-            for _ in range(self.ITEMS):
+            for _ in range(self.SMALL_ITEMS):
                 item = C()
                 items.append(item)
             return items