Change warnings.warn to logging.warning

[Documentation](https://docs.python.org/3/library/logging.html)

This will look nicer. Also see [this stackoverflow question/answer](https://stackoverflow.com/questions/9595009/warnings-warn-vs-logging-warning/14762106#14762106); still seems an appropriate use of the logging functionality, since it's used as notes/informative, rather than something that the user strictly should change.