Change print to logging.warning in ini_reader.

[rplzzz]

Fixes #49

@crvernon FWIW, I wasn't able to run the test suite locally because Xanthos doesn't install cleanly under python-3.7.7 on our system. The error is:

error in xanthos setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Invalid requirement, parse error at "'://git-l'"

No time to track down the cause right now, unfortunately.