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/ntpath.py

@@ -481,27 +481,28 @@ def normpath(path):
 
 
 # Return an absolute path.
-def abspath(path):
-    """Return the absolute version of a path"""
-    try:
-        from nt import _getfullpathname
-    except ImportError: # Not running on Windows - mock up something sensible.
-        global abspath
-        def _abspath(path):
-            if not isabs(path):
-                path = join(os.getcwd(), path)
-            return normpath(path)
-        abspath = _abspath
-        return _abspath(path)
+try:
+    from nt import _getfullpathname
 
-    if path: # Empty path must return current working directory.
-        try:
-            path = _getfullpathname(path)
-        except WindowsError:
-            pass # Bad path - return unchanged.
-    else:
-        path = os.getcwd()
-    return normpath(path)
+except ImportError: # not running on Windows - mock up something sensible
+    def abspath(path):
+        """Return the absolute version of a path."""
+        if not isabs(path):
+            path = join(os.getcwd(), path)
+        return normpath(path)
+
+else:  # use native Windows method on Windows
+    def abspath(path):
+        """Return the absolute version of a path."""
+
+        if path: # Empty path must return current working directory.
+            try:
+                path = _getfullpathname(path)
+            except WindowsError:
+                pass # Bad path - return unchanged.
+        else:
+            path = os.getcwd()
+        return normpath(path)
 
 # realpath is a no-op on systems without islink support
 realpath = abspath