spanlib can be installed with conda in Python 2.7, but not 3.x

[jypeter]

@stefraynaud I had no problem installing spanlib in the Python 2.7 version of CDAT 8.1, but I'm having the same kind of endless guru meditation when trying to install into the Python 3 version of CDAT 8.1 as what I described for vacumm in VACUMM/vacumm#15

I guess the current/standard version of spanlib is for Python 2.7 only

You may want to mention this on https://github.com/stefraynaud/spanlib

Note that installing with conda is not mentioned on the spanlib pages. For Python 2.7, I have successfully used conda install -n my_python_env -c stefraynaud -c conda-forge spanlib. You may want to mention this somewhere. I don't remember if I found out about this by myself a long time ago, or if you told me about it