Fix highlight's linenothreshold for Sphinx >=1.8+ & <2.0
Fix highlight's linenothreshold for Sphinx >=1.8+ & <2.0

Replace Sphinx's HighlightLanguageTransform with our own just for Sphinx
versions between 1.8.0 & less than 2.0.0 as these versions have broken
the linenothreshold setting in the highlight directive.

This code can be removed when we drop support for Python 2

Fixes #721

rst2pdf/rst2pdf,rst2pdf/rst2pdf