Warnings are written to stdout instead of stderr

[SECtim]

The README says

Errors and warnings are only printed on the console (into the standard error output stream)

However, this is not the case. For example, https://github.com/iluxonchik/rfc-bibtex/blob/84bca572bbbe9a65ccb5ee1acd84d1b9f09f97d5/rfc_bibtex/rfc_bibtex.py#L127-L129 seems to print to stderr, but doesn't: https://github.com/iluxonchik/rfc-bibtex/blob/84bca572bbbe9a65ccb5ee1acd84d1b9f09f97d5/rfc_bibtex/utils.py#L9-L10

This is annoying when adding bib entries to a bib file, e.g. rfcbibtex draft-ietf-httpbis-rfc6265bis >> specs.bib, because the warning ends up in the bib file.