Address SF bug 519621: slots weren't traversed by GC.

While I was at it, I added a tp_clear handler and changed the
tp_dealloc handler to use the clear_slots helper for the tp_clear
handler.

Also tightened the rules for slot names: they must now be proper
identifiers (ignoring the dirty little fact that <ctype.h> is locale
sensitive).

Also set mp->flags = READONLY for the __weakref__ pseudo-slot.

Most of this is a 2.2 bugfix candidate; I'll apply it there myself.

Lib/test/test_descr.py

@@ -1060,6 +1060,45 @@ def slots():
     vereq(x.b, 2)
     vereq(x.c, 3)
 
+    # Make sure slot names are proper identifiers
+    try:
+        class C(object):
+            __slots__ = [None]
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "[None] slots not caught"
+    try:
+        class C(object):
+            __slots__ = ["foo bar"]
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "['foo bar'] slots not caught"
+    try:
+        class C(object):
+            __slots__ = ["foo\0bar"]
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "['foo\\0bar'] slots not caught"
+    try:
+        class C(object):
+            __slots__ = ["1"]
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "['1'] slots not caught"
+    try:
+        class C(object):
+            __slots__ = [""]
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "[''] slots not caught"
+    class C(object):
+        __slots__ = ["a", "a_b", "_a", "A0123456789Z"]
+
     # Test leaks
     class Counted(object):
         counter = 0    # counts the number of instances alive
@@ -1094,6 +1133,18 @@ def slots():
     del x
     vereq(Counted.counter, 0)
 
+    # Test cyclical leaks [SF bug 519621]
+    class F(object):
+        __slots__ = ['a', 'b']
+    log = []
+    s = F()
+    s.a = [Counted(), s]
+    vereq(Counted.counter, 1)
+    s = None
+    import gc
+    gc.collect()
+    vereq(Counted.counter, 0)
+
 def dynamics():
     if verbose: print "Testing class attribute propagation..."
     class D(object):