Correct the apparent refleak in test_io:

When cls is an ABCMeta, every call to isinstance(x, cls)
records type(x) in the cls._abc_cache of cls_abc_negative_cache.
So we clear these caches at the end of the test.

inspect.isabstract() is not the correct test for all ABCs, because there is no @abstractmethod in io.py (why?)
isinstance(cls, ABCMeta) would be more exact, but it fails with an infinite recursion.
So I used a hack to determine whether a class is an ABCMeta.

The true correction would be to turn cls._abc_cache &co into a WeakSet, as py3k does.
But classic classes are not weak referenceable...

Of course, this change should not be merged into the py3k branch.

Lib/test/regrtest.py

@@ -128,7 +128,6 @@ import sys
 import time
 import traceback
 import warnings
-from inspect import isabstract
 
 # I see no other way to suppress these warnings;
 # putting them in test_grammar.py has no effect:
@@ -630,7 +629,7 @@ def cleanup_test_droppings(testname, verbose):
 
 def dash_R(the_module, test, indirect_test, huntrleaks):
     # This code is hackish and inelegant, but it seems to do the job.
-    import copy_reg, _abcoll
+    import copy_reg, _abcoll, io
 
     if not hasattr(sys, 'gettotalrefcount'):
         raise Exception("Tracking reference leaks requires a debug build "
@@ -641,8 +640,10 @@ def dash_R(the_module, test, indirect_test, huntrleaks):
     ps = copy_reg.dispatch_table.copy()
     pic = sys.path_importer_cache.copy()
     abcs = {}
-    for abc in [getattr(_abcoll, a) for a in _abcoll.__all__]:
-        if not isabstract(abc):
+    modules = _abcoll, io
+    for abc in [getattr(mod, a) for mod in modules for a in mod.__all__]:
+        # XXX isinstance(abc, ABCMeta) leads to infinite recursion
+        if not hasattr(abc, '_abc_registry'):
             continue
         for obj in abc.__subclasses__() + [abc]:
             abcs[obj] = obj._abc_registry.copy()
@@ -679,7 +680,7 @@ def dash_R_cleanup(fs, ps, pic, abcs):
     import gc, copy_reg
     import _strptime, linecache, dircache
     import urlparse, urllib, urllib2, mimetypes, doctest
-    import struct, filecmp, _abcoll
+    import struct, filecmp
     from distutils.dir_util import _path_created
 
     # Restore some original values.
@@ -693,13 +694,10 @@ def dash_R_cleanup(fs, ps, pic, abcs):
     sys._clear_type_cache()
 
     # Clear ABC registries, restoring previously saved ABC registries.
-    for abc in [getattr(_abcoll, a) for a in _abcoll.__all__]:
-        if not isabstract(abc):
-            continue
-        for obj in abc.__subclasses__() + [abc]:
-            obj._abc_registry = abcs.get(obj, {}).copy()
-            obj._abc_cache.clear()
-            obj._abc_negative_cache.clear()
+    for abc, registry in abcs.items():
+        abc._abc_registry = registry.copy()
+        abc._abc_cache.clear()
+        abc._abc_negative_cache.clear()
 
     # Clear assorted module caches.
     _path_created.clear()