textproc/py-mkdocstrings: Update to 0.24.0

• Remove superfluous pytest dependency from TEST_DEPENDS as it's already set via USE_PYTHON=pytest while I'm here.

Changelog:

https://github.com/mkdocstrings/mkdocstrings/releases/tag/0.24.0
(cherry picked from commit f7d874c638020a951daa48e2311600e6858e002f)