Use decorator to classmethod.

[dongweiming]

This script requires Python 2.6 or greater. So can use decorator
instead re-bind function

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow virtualenv has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master.

If you do nothing, this Pull Request will be automatically migrated by @BrownTruck for you.

If you would like to retain control over this pull request then you should resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to migrate this Pull Request yourself, here is an example message that you can copy and paste:

This script requires Python 2.6 or greater. So can use decorator
 instead re-bind function

---

*This was migrated from pypa/virtualenv#547 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

If this pull request is no longer needed, please feel free to close it.

[dongweiming]

Ok i will resubmit this.