sympyで「なぜ三角形の内角の和は180度？」を証明できなかった

(参考)なぜ三角形の内角の和は180度？
http://qiita.com/tomo64/items/d25c0435dfcc62154a43

from sympy import *
var('x1 y1 x2 y2 x3 y3')
# P1=Point(x1,y1)
# P2=Point(x2,y2)
# P3=Point(x3,y3)
# print( Polygon(P1,P2,P3).angles[P2] + Polygon(P2,P3,P1).angles[P3]  + Polygon(P3,P1,P2).angles[P1])
# ValueError: Can't determine orientation
x1=0
y1=0
x2=100
y2=0
x3=100
y3=100
P1=Point(x1,y1)
P2=Point(x2,y2)
P3=Point(x3,y3)
print( Polygon(P1,P2,P3).angles[P2] + Polygon(P2,P3,P1).angles[P3]  + Polygon(P3,P1,P2).angles[P1])
# pi