packages/lang/Python/README.md is out-of-date

Maintainer: @jefferyto

The "Host-side Python packages for build" section of packages/lang/Python/README.md contains the following language:

These can be installed via pip and ideally they should only be installed like this, because it's a bit simpler than running them through the OpenWrt build system.

According to these instructions, installing a host Python package relies upon on a file specifying the requirements for that package being present in host-pip-requirements directory; however, the contents of that directory were removed in 3ee4e7297cf07b644bac3dfafc508da5f31bf63d. The new python-setuptools-scm Makefile does not seem to use pip at all, but rather downloads source tarballs from Pypi.

I have two questions:

1. Is the new approach to installing host Python packages to write a Makefile in the style of python-setuptools-scm?
2. Assuming the answer to the question above is yes, if I attempt to write a Makefile for a new host Python package, will I also be required to write a Makefile for each Python package that package depends upon?

Start with packages/lang/Python/README.md, the host-pip-requirements history at commit 3ee4e7297cf07b644bac3dfafc508da5f31bf63d, and the lang/python/python-setuptools-scm/Makefile. Confirm the current host-package workflow and dependency expectations with the relevant package metadata. Done means the README accurately documents the supported approach and required dependency handling.