Computing the sentiment score of a sentence
TSV parsing
You should not need to do this yourself. The csv module accepts a delimiter that you can set to '\t'.
Context management
You use with in one out of three file opens - the other two would benefit.
Inner list
This:
set([line.rstrip() for line in lexiconpos])

can be
{line.rstrip() for line in lexiconpos}

The set literal and removal of the inner list will both express this better.