textproc/html2text: Update CONFLICTS

The CONFLICTS definition did only match conflicting packages built
for Python-2.7, not for later Python versions.

I have not checked whether CONFLICTS_INSTALL would suffice as used
in the conflicting ports. The PORTREVISION has not been bumped since
this parameter is relevant for port building, less so for installing
from a package (which detects the conflicting files).

Approved by: bapt (maintainer)