Simplify dark-mode handling for PyPI-uploaded README
Use a responsive SVG with CSS media queries instead of the `<picture>` element,
and simplify the transform script to use URL replacement instead of block
replacement. PyPI doesn't support CSS media queries, so we swap in a light-only
SVG for PyPI deployments.
Restore OpenSUSE system test in CI
Re-enable the `system-test-opensuse` job that was disabled in #13818
due to CI flakes (see #13811). The test validates Python installation
on openSUSE Tumbleweed.