Fix issue with running on python 2.6

I'm on python 2.6 (CentOS6) and the resulting executable fails to run with this error..

```pytb
Traceback (most recent call last):
  File "/usr/lib/python2.6/runpy.py", line 122, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.6/runpy.py", line 34, in _run_code
    exec code in run_globals
  File "./devtool.sz/__main__.py", line 30, in <module>
  File "./devtool.sz/zipsite.py", line 148, in addsitedir
AttributeError: ZipFile instance has no attribute '__exit__'
```
...because the ability to use a ZipFile as a context-manager wasn't added until 2.7 according to http://docs.python.org/2/library/zipfile.html#zipfile-objects

This change fixed it for me..