tags2set: let the logging API format the string
tags2set: fix a few pylint issues

Fixes an overly long line and missing spaces around operators. No
functional change and tests still seem to pass.
setup.py: add a test target
tags2set: remove unnecessary global variable 'user'
tags2set: createSet is semi-private, remove global

Rename createSet to _createSet as it is not called outside this module.
Remove the global variable existingSets, pass it to the function as
needed.