New comment by ahesford on void-packages repository

https://github.com/void-linux/void-packages/pull/26883#issuecomment-737648438

Comment:
Comment added to `update` restriction on `python-packaging`, created a separate `python3-pep517` build style and updated the manual. This simplified some things, and I moved the `pytest` adoption in `python3-module` to #26901 for separate consideration.

While doing a separate build style, I decided to allow `$make_build_target` to override the default `$PWD` target when necessary, and uesd `$make_install_target` instead of a new variable to select the wheel used for installation. This avoids the need for an xlint update.