Install different python versions in separate environments

[nathanlyons]

Addresses #20.

A synopsis of this issue and solution from @mcflugen in landlab/landlab#633:

This has to do with a change with conda/Miniconda3 that no longer allows installing a different version of Python into the root environment. To fix this, I changed things so that we create a separate environment (with the appropriate Python version) to run our tests in.

@mcflugen,

• I set the base of this pull request to next in order to get the new tutorials passing.
• I may have created a branch, new by accident. I see this branch in my branches list. Before I delete the new branch, I wish to verify with you that the new branch should indeed not be a thing.