MFH: r448305

net/py-pcs: Limit USES=python (actually)

Actually change the value of USES=python:<version-spec> that was
supposed to land in ports r448303

It was reverted during QA and the final diff was not reviewed prior to commit.

Pointyhat me.

PR: 219325
Approved by: portmgr (blanket)

Approved by: portmgr (blanket)