Modernize to Py2.6+3

examples/curve-pot.py

examples/curve.py

examples/fourier.py

sumpy/__init__.py

sumpy/assignment_collection.py

sumpy/codegen.py

sumpy/e2e.py

sumpy/e2p.py

sumpy/expansion/__init__.py

sumpy/expansion/local.py

sumpy/expansion/multipole.py

sumpy/fmm.py

sumpy/kernel.py

sumpy/p2e.py

sumpy/p2p.py

sumpy/qbx.py

sumpy/symbolic.py

sumpy/tools.py

sumpy/visualization.py

test/test_fmm.py