CLN/ENH: Merge compat with py3compat and six

For simplicity, move the relevant portions of six into
compat and combine py3compat and compat together (given the distinctions
were relatively arbitrary already). Also switch to importing u to make
things cleaner.

CLN: Transition six --> compat

Simplify compat imports.

fix imports

remove threading from python 3
DOC/ENH: Deprecate iterkv + add docs