Fix resource leaks, typo, Hearst pattern, and scorer span pairing

[Chessing234]

Summary

• Close unclosed file handles in read_full_med_mentions(), read_ner_from_tsv(), and evaluate_ner() using context managers
• Fix typo constuctor → constructor in CandidateGenerator error message
• Fix which_look_like Hearst pattern to use hypernym instead of hyponym at the end of the pattern
• Fix PerClassScorer span pairing bug caused by zipping a set with a list

Issues addressed

Fixes #597, #599, #601, #595, #592

Test plan

• Run pytest tests/test_data_util.py tests/test_per_class_scorer.py
• Verify Hearst pattern matching for phrases like "cells which look like fibroblasts"

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