Kokoro (Windows): Switch to python3
Kokoro (Windows): Switch to python3

SPIRV-Tools now requires python3.

Bug: b/135609394
Change-Id: Ib33ddb57b0bdfa87abed94de6dbfb91eadbc31bf

google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader