Keep imports alphabetically sorted and their order homogeneous across
Python source files.
The isort project has more feature and is more active than the
flake8-import-order plugin.
Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.
They are sometimes failed with python3.5 because the order of singledispatch
functions is not stable on python 3.5. This uses comparision via "in"
keyword to check the signature of singledispatch functions stably.
