bug#40029: Preventing automatic python2 transformation of some packages

[Marius Bakke <mbakke@fastmail.com>]

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Guix,

'python2-sphinx' is obsolete and increasingly a maintenance burden,
because we need to keep special versions around just to make it build.

The only reason we have this package is because it gets pulled in
automatically when using (package-with-python2 ...) on a Python 3
package that has 'python-sphinx' in inputs.

Most, or all, Python packages do not use Sphinx as runtime library, but
only calls its executables to generate documentation.  Thus, we do not
actually need the Python 2 variant.

I tried fixing this in (guix build-system python) by only transforming
packages whose names start with 'python-', and renaming 'python-sphinx'
to just 'sphinx'.  It was a clever one-liner until I realized that
Python 2 packages now got a variant of 'sphinx' that's built with Python
3 (yay!), but all the *inputs* of this Sphinx variant were transformed
into Python 2!  So it did not actually build in the end.

I'm mostly venting here, but filing the bug so we can target it for the
next 'core-updates' round.  Ideas welcome.

There are some other improvements I'd like to see in
python-build-system, such as automatically calling 'pytest' instead of
the deprecated 'python setup.py test' when pytest is available.

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