Removing Price Pfister widespread bath sink outlet Model WL8-5 (mfg 2005)
That whole tee piece needs to come off, then there is a hex shaped nut under it that holds the spout assembly to the sink. It's often a 2-person job because the force needed to remove the tee piece and nut after its been on there so long can cause the spout itself to turn.
So... someone needs to hold the spout up top while another removes the plug (which looks like it has been wrenched on and drilled into) then the tee piece. Then the nut needs to be removed. It's a tough jog and a variety of tools (Channellocks, pipe wrench, hammer to tap with, plumber's sockets, etc.) are helpful. Also helpful would be to soak it with penetrating oil first. I will warn you that it would not be uncommon if destructive means were necessary...