Revert "Whitelist external netaddr requirement"

This reverts commit de59c55e77ead72df42c0445c09cf4d63131f6bd.

The netaddr developers have agreed to go back to uploading releases
to PyPI, so this workaround is no longer necessary. For details see
https://github.com/drkjam/netaddr/issues/57#issuecomment-31796111 .

Change-Id: Ieeb3e8e549d9e4b53dba0165277fa2423d2eba01
Merge "Revert "Whitelist external netaddr requirement""