Replace references to 'log.warn' with 'log.warning'

to remove some `DeprecationWarning`s from the logs
as the former method as been deprecated since Python 3.3 [1][2].

- [1] https://github.com/python/cpython/blob/3.3/Lib/logging/__init__.py#L1252-L1253
- [2] https://stackoverflow.com/a/15655674