remove workarounds to support Python 2

[carandraug]

microscope no longer works in Python 2, so we could remove some historical workarounds to make it work, and drop the dependency on six.

[carandraug]

I removed the usage of six where I found it, and all the workarounds in setup.py. See 1955da0 (or my remove-python2-workarounds branch)

The only thing we still have that was used to support python 2and 3 at the same time is the DTYPES stuff in microscope.devices which I'm not so familiar with. Can someone take a look at removing it?

[carandraug]

Now all done.