PEP3113 compliance for theano.sandbox, theano.sparse.sandbox, and theano/scalar/basic_scipy.py. Now only sparse/basic.py and scalar/basic.py to do.

Allow to move to the GPU Composite op that have non float32 in them but float32 in input and output.

test that we con't move Composite with float64 in them to the gpu and that we don't move elemwise comparaison to the gpu even if the output is cast to float32.