Mathtext distinguishes italic and normal font

Matplotlib's lightweight TeX expression parser (usetex=False) now distinguishes between italic and normal math fonts to closer replicate the behaviour of LaTeX. The normal math font is selected by default in math environment (unless the rcParam mathtext.default is overwritten) but can be explicitly set with the new \mathnormal command. Italic font is selected with \mathit. The main difference is that italic produces italic digits, whereas normal produces upright digits. Previously, it was not possible to typeset italic digits. Note that normal now corresponds to what used to be it, whereas it now renders all characters italic. Important: In case the default mathematics font is overwritten by setting mathtext.default: it in matplotlibrc, it must be either commented out or changed to mathtext.default: normal to preserve its behaviour. Otherwise, all alphanumeric characters, including digits, are rendered italic.

One difference to traditional 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 nor in Matplotlib.