textproc/py-m2r: Update to 0.1.6

• Update PORTVERSION and distinfo checksum (0.1.6)
• Add LICENSE_FILE (LICENSE)
• Add comment on supported Python versions for the future

https://github.com/miyakogi/m2r/blob/master/CHANGES.md

PR: 219762
Submitted by: John W. O'Brien <john saltant com> (maintainer)