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/threaded_import_hangers.py

@@ -0,0 +1,42 @@
+# This is a helper module for test_threaded_import.  The test imports this
+# module, and this module tries to run various Python library functions in
+# their own thread, as a side effect of being imported.  If the spawned
+# thread doesn't complete in TIMEOUT seconds, an "appeared to hang" message
+# is appended to the module-global `errors` list.  That list remains empty
+# if (and only if) all functions tested complete.
+
+TIMEOUT = 10
+
+import threading
+
+import tempfile
+import os.path
+
+errors = []
+
+# This class merely runs a function in its own thread T.  The thread importing
+# this module holds the import lock, so if the function called by T tries
+# to do its own imports it will block waiting for this module's import
+# to complete.
+class Worker(threading.Thread):
+    def __init__(self, function, args):
+        threading.Thread.__init__(self)
+        self.function = function
+        self.args = args
+
+    def run(self):
+        self.function(*self.args)
+
+for name, func, args in [
+        # Bug 147376:  TemporaryFile hung on Windows, starting in Python 2.4.
+        ("tempfile.TemporaryFile", tempfile.TemporaryFile, ()),
+
+        # The real cause for bug 147376:  ntpath.abspath() caused the hang.
+        ("os.path.abspath", os.path.abspath, ('.',)),
+        ]:
+
+    t = Worker(func, args)
+    t.start()
+    t.join(TIMEOUT)
+    if t.isAlive():
+        errors.append("%s appeared to hang" % name)