These files were added in #29287 (SPKG type: Make "normal/script/pip" orthogonal to "base/standard/optional/experimental") for all pip packages.

We record "abstract" version requirements in files build/pkgs/SPKG/install-requires.txt - this is #30719.

In this ticket, we additionally record "concrete" versions for all Python packages in build/pkgs/SPKG/requirements.txt.

(cf. https://packaging.python.org/discussions/install-requires-vs-requirements/)

Some scripts need updating to make sure that checksums.ini wins over requirements.txt - to keep normal packages normal.

Depends on #30719

CC: @slel @jhpalmieri @orlitzky

Component: build

Issue created by migration from https://trac.sagemath.org/ticket/30024