* Split packaging metadata into an extras_require.json file instead of using
the pep517 and the pip modules directly. This was causing build failures if
not using a virtualenv and/or building without internet access.
* Add an external tool reference for GNU Guix (lzip).
* Drop an external tool reference for GNU Guix (pedump).
* Split inline Python code in shell script to generate test dependencies to a
separate Python script.
* No need for "from __future__ import print_function" import in setup.py
anymore.
* Comment and tidy the new extras_require.json handling.
OBS-URL: https://build.opensuse.org/package/show/devel:languages:python/diffoscope?expand=0&rev=82
