Month names not set as text when using usetex

[maxnoe]

Bug report

From the image in the release notes for matplotlib 3.4 here:
https://matplotlib.org/stable/users/whats_new.html#date-formatters-now-respect-usetex-rcparam

The dateformatter does not use text mode for the month names but sets them like $Aug$. Which is bad typography, since it will be typeset as the mathematical expression A * u * g, not the text Aug.

[anntzer]

Currently the formatter wraps text in \mathdefault (so things work with usetex=False), but mathdefault is a matplotlib-only thing and we just strip it out for usetex. In #18520 I argued that we should perhaps map it to amsmath's \text instead, which would fix that.

[smartlixx]

If amsmath's \text is not introduced in the near future, we can modify _wrap_in_tex function to differentiate alphabets and other characters, leaving alphabets out from \mathdefault. This way is a little clumsy, but it works. A PR can be open with this approach.

[tacaswell]

@maxnoe Was this a change from 3.3? silly question, it is a new feature in 3.4 🐑