textproc/py-rst2html: Fix conflict with textproc/py-docutils

docutils 0.13.1 release included a new rst2html5 frontend, adding
rst2html5{.py} to LOCALBASE/bin. This conflicts with files already
packages with textproc/py-rst2html [1][2], resulting in a packging error.

Also affected (not currently packaged in FreeBSD Ports), is the
rst2html5-tools PyPI package [2].

Rename the conflicting script accordingly.

While I'm here:

• Fix *_DEPENDS versions to match upstream
• Remove unecessary limit to Python 2.x
• Enable concurrent installation

[1] https://bitbucket.org/andre_felipe_dias/rst2html5/issues/27/
[2] https://github.com/getnikola/nikola/issues/2657
[3] https://github.com/marianoguerra/rst2html5/issues/87

PR: 228414
Reported by: roflik (via IRC)
Approved by: koobs (python, maintainer)
MFH: 2018Q2