[mesh] A small fix for mesh loop syntax in python frontend

[BillXu2000]

Related issue = #3608

import taichi as ti, argparse

parser = argparse.ArgumentParser()
parser.add_argument('--json')
args = parser.parse_args()

outer = 'faces'
inner = 'edges'

ti.init(arch=ti.cuda)
vecf = ti.types.vector(3, ti.f32)

mesh = ti.TetMesh()

mesh.faces.link(mesh.edges)
mesh.faces.link(mesh.verts)
bunny = mesh.build(ti.Mesh.load_meta(args.json))

faces = bunny.faces

@ti.kernel
def get_off():
    for i in faces:
        pass
get_off()

make for i in faces: in ti.kernel work.

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[CLAassistant]

All committers have signed the CLA.

[g1n0st]

LGTM!

[BillXu2000]

I wonder why Apple M1 Tests failed here...
Besides, I push these commits with a new email, and CLA becomes a problem here now. How can I fix this?
@ailzhang @k-ye