PyPI packages are mandated to have a setup.py file, which contains a listing
of the required dependencies. The setuptools/distutils machinery embed
metadata in the archives they produce, which contains this information. There
is no need nor gain to collect the requirements from a "requirements.txt"
file, as it is not the true record of dependencies for PyPI packages and may
contain extraneous requirements or not exist at all.
* guix/import/pypi.scm (guess-requirements): Update comment.
[guess-requirements-from-source]: Do not attempt to parse the file
requirements.txt. Streamline logic.
* tests/pypi.scm (test-requires.txt): Rename from test-requirements, to hint
at the file being tested.
("pypi->guix-package"): Adapt so that the fake package contains a requires.txt
file rather than a requirements.txt file.
("pypi->guix-package, wheels"): Likewise.