Tweak output from maxima

Tweak output from maxima

Maxima uses \it as a markup for multiletter variables. However,
it has been reported that since texlive 2016 using \it in math
mode produces an error, even though I was not able to reproduce.
Anyway, this can be avoided by replacing the old-style construct
"{\it ...}" with the new-style one "\mathit{...}".
The problem has also been reported upstream:
https://sourceforge.net/p/maxima/bugs/3181/
but this workaround will hold whatever the resolution.