Mathtext distinguishes *italic* and *normal* font

Matplotlibs lightweight TeX expression parser (`usetex=False`) now distinguishes between *italic* and *normal* math fonts to closer replicate the behaviour of LaTeX.

Italic font is selected with `\mathit`, whereas the normal math font is selected by default in math environment but can be explicitly set with the new `\mathnormal` command. Previously, it was not possible to typeset italic digits.

The main difference is that, *italic* produces italic digits, whereas *normal* produces upright digits.

One difference to historical LaTeX is that LaTeX further distinguishes between *normal* (`\mathnormal`) and *default math*, where the default uses roman digits and normal uses oldstyle digits. This distinction is no longer present with modern LaTeX engines and unicode-math.