Make export_tarball.py work with older Pythons.

tools/export_tarball/export_tarball.py

Index: tools/export_tarball/export_tarball.py
diff --git a/tools/export_tarball/export_tarball.py b/tools/export_tarball/export_tarball.py
index 607f478f8810b4e7db11a9e83e4f5129c1b42009..6715f110fb92d43108150e4cfe32c8524f4ab23c 100644
--- a/tools/export_tarball/export_tarball.py
+++ b/tools/export_tarball/export_tarball.py
@@ -16,9 +16,6 @@ export_tarball.py /foo/bar
 The above will create file /foo/bar.tar.bz2.
 """
 
-from __future__ import with_statement
-
-import contextlib
 import optparse
 import os
 import sys
@@ -74,9 +71,12 @@ def main(argv):
 
     return False
 
-  with contextlib.closing(tarfile.open(output_fullname, 'w:bz2')) as archive:
+  archive = tarfile.open(output_fullname, 'w:bz2')
+  try:
     archive.add(GetSourceDirectory(), arcname=output_basename,
                 exclude=ShouldExcludePath)
+  finally:
+    archive.close()
 
   return 0