Cannot build for Python 3.7

[swainn]

I can't get gsshapy to build for Python 3.7 on conda forge due to one it's dependencies not being built for 3.7 (rapidpy). There may be other issues too.

See: https://dev.azure.com/conda-forge/feedstock-builds/_build/results?buildId=38198

For now, it'll be configured to only build Python 3.6, but we should address the roadblocks at some point. One way around this would be to make rapidpy and other similar dependencies optional. It doesn't seem to get used in the core ORM.

[snowman2]

Also needs: conda-forge/pangaea-feedstock#4 (comment)