Don't use newtxmath with xetex or luatex

[zackw]

Using newtxmath with xetex or luatex produces PDFs that are missing /ToUnicode annotations for the math symbol fonts (txsy*); the most important practical reason this is a problem, is that (depending on how complex your math is and howclever your PDF viewer is) it may make it impossible to copy equations out of a document.

The maintainer of the libertine CTAN package recommends using \usepackage[libertine]{newtxmath} only for pdftex, and

\usepackage{unicode-math}
\setmathfont[Scale=MatchUppercase]{libertinusmath-regular.otf}

instead with xetex or luatex. Note that libertinusmath-regular.otf is one of the Libertinus fonts, so it might make sense to switch to them, generally, at the same time, as suggested in #398.