Support mdformat 0.7.20

[antazoey]

I am getting

ImportError: cannot import name 'cached_textwrapper' from partially initialized module 'mdformat._util' (most likely due to a circular import) (/Users/jules/virtualenvs/ape/lib/python3.12/site-packages/mdformat/_util.py)

on mdformat 0.7.20

[antazoey]

So 0.7.20 was yanked and they fixed the problem in mdformat.. however, it is still more performant to use type checking imports so it is a win! thank you ◡̈