SF bug #1473760 TempFile can hang on Windows.

Python 2.4 changed ntpath.abspath to do an import
inside the function.  As a result, due to Python's
import lock, anything calling abspath on Windows
(directly, or indirectly like tempfile.TemporaryFile)
hung when it was called from a thread spawned as a
side effect of importing a module.

This is a depressingly frequent problem, and
deserves a more general fix.  I'm settling for
a micro-fix here because this specific one accounts
for a report of Zope Corp's ZEO hanging on Windows,
and it was an odd way to change abspath to begin
with (ntpath needs a different implementation
depending on whether we're actually running on
Windows, and the _obvious_ way to arrange for that
is not to bury a possibly-failing import _inside_
the function).

Note that if/when other micro-fixes of this kind
get made, the new Lib/test/threaded_import_hangers.py
is a convenient place to add tests for them.

Lib/test/test_threaded_import.py

@@ -6,7 +6,7 @@
 # randrange, and then Python hangs.
 
 import thread
-from test.test_support import verbose, TestSkipped
+from test.test_support import verbose, TestSkipped, TestFailed
 
 critical_section = thread.allocate_lock()
 done = thread.allocate_lock()
@@ -25,6 +25,23 @@ def task():
     if finished:
         done.release()
 
+def test_import_hangers():
+    import sys
+    if verbose:
+        print "testing import hangers ...",
+
+    from test import threaded_import_hangers
+
+    try:
+        if threaded_import_hangers.errors:
+            raise TestFailed(threaded_import_hangers.errors)
+        elif verbose:
+            print "OK."
+    finally:
+        # In case this test is run again, make sure the helper module
+        # gets loaded from scratch again.
+        del sys.modules['test.threaded_import_hangers']
+
 # Tricky:  When regrtest imports this module, the thread running regrtest
 # grabs the import lock and won't let go of it until this module returns.
 # All other threads attempting an import hang for the duration.  Since
@@ -53,5 +70,7 @@ def test_main():        # magic name!  see above
             print "OK."
     done.release()
 
+    test_import_hangers()
+
 if __name__ == "__main__":
     test_main()