These files:

• serve as a marker that an SPKG is a Python package (for Add configure option --with-sage-venv=SAGE_VENV to create venv there instead of in SAGE_LOCAL #29013)
• may include comments that explain why we reject certain versions
• provide version constraints (lower and upper bounds)
• as such, are preparation for Meta-ticket: ./configure --enable-system-site-packages (optionally use system Python packages) #29023: Meta-ticket: In a python 3 build, optionally use system Python packages
• can be used to generate other metadata such as src/Pipfile, src/setup.cfg [install_requires] (sage_bootstrap: Add command "sage -package list --output={install-requires,requirements,pipfile,debian,....}" #29041) in the bootstrap phase, to avoid duplicating this information in various places

CC: @tobiasdiez @isuruf @kiwifb @antonio-rojas @dimpase

Component: build

Author: Matthias Koeppe

Branch/Commit: 6ec00dd

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/30719