net/py-libnet: Limit to 2.x (Does not support Python 3)

This (upstream) package does not appear to support (intentionally or otherwise)
Python 3. It also fails to build with Python 3.x with a SyntaxError.

This change corrects the USES=python:<version-spec> support declaration
accordingly.

PR: 219324
Reported by: Johannes Jost Meixner
Approved by: portmgr (blanket)
MFH: 2017Q3