Set mathtext.fontset per element

How can we set [mathtext font](https://matplotlib.org/3.2.1/tutorials/text/mathtext.html) without manipulating RC per [Text](https://matplotlib.org/3.2.2/api/text_api.html#matplotlib.text.Text) element. Text objects cannot take in this parameter, as apparently the only way to set `"mathtext.fontset"="Serif"` is to make it global (not per some Text element).  

