Remove use of six module

[drewj-tp]

The six module is great for projects that need to span py2 and py3. But that has not been the case for a long time.

Let's

• drop if from our dependencies
• replace instances of six.* with the py3 variants (e.g., six.string_types with str)
• remove imports of six across the project

[john-science]

Oh, yeah, that can definitely go.