Conda says that siphon and Python 3.5 are in conflict with each other on both linux and mac. It looks like you have 3.5 binaries on anaconda.org, so maybe something specific to how you built them?

Siphon is a collection of Python utilities for downloading data from Unidata data technologies. See our support page for ways to get help with Siphon. Siphon follows semantic versioning in its version ...