Remove usage of six
Removes all usage of six from the codebase.
Since this involves replacing six.ensure_string and six.ensure_binary, this diff also adopts the convention that we use explicit .decode("utf-8") and .encode("utf-8") everywhere, although I am inclined to change this to bare .decode() and .encode() everywhere since utf-8 is always the default. Thoughts?
Test Plan: unit
Reviewers: alangenfeld, max
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5944