security/zeronet: Limit to Python 2.7

Zeronet doesn't support Python 3 [1]. Limit to compatible version(s) (2.7)
accordingly.

[1] https://github.com/HelloZeroNet/ZeroNet/issues/149

PR: 220302
Reported by: Peter Zuidema <peter icebear net>
Submitted by: Yuri Victorovich <yuri rawbw com> (maintainer)
MFH: 2017Q3