particles: Import SpriteAnim in Particles.py

Fixes #544
Closes #545

direct/src/particles/Particles.py

@@ -22,6 +22,7 @@ from panda3d.physics import RingEmitter
 from panda3d.physics import SphereSurfaceEmitter
 from panda3d.physics import SphereVolumeEmitter
 from panda3d.physics import TangentRingEmitter
+from panda3d.physics import SpriteAnim
 
 from . import SpriteParticleRendererExt