Corpus.py needs updating for YAML>5

[pmi123]

corpus.py line 38 needs to change from

return yaml.load(data_file)

to

return yaml.load(data_file, Loader=yaml.FullLoader)

per this update to yaml:
https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

[WeileiZeng]

yaml.safe_load(data_file) also works