textproc/py-mkdocs: Update to 1.4.2

• Switch to PEP-517 build framework.

Changelogs since 1.4.0:

https://github.com/mkdocs/mkdocs/releases/tag/1.4.2
https://github.com/mkdocs/mkdocs/releases/tag/1.4.1

MFH: No (PEP-517 support not present in 2023Q1 branch)