testing/m2r2 will probably be broken by py3-mistune 2.0.2 upgrade

In !33321 (merged), I'm upgrading py3-mistune to version 2.0.2. This will probably break m2r2 as it explicitly depends on Mistune 0.8.4. However, Mistune 0.8.4 and 2.0.2 cannot trivially be installed in parallel and I'm not interested in maintaining Mistune 0.8.4. I also don't want to hold this MR for now since Alpine 3.16 is releasing soon and I want both Giara and Moment to work in Alpine 3.16. m2r2 can be fixed later since it's in testing and as such won't be part of Alpine 3.16 anyway.

CC @Saijin-Naib since you maintain this package