remove python-coverage dependency

While updating my system today, I noticed that I had python-coverage installed, which surprised me, as I have all my Python dev packages installed in venvs and not as system packages via pacman. After checking the dependency graph, I only found inkscape as the depending package.

python-coverage was added as a runtime dependency in 2908b95f

Considering that python-coverage is usually not a runtime dependency, but only used for running tests, I built the PKGBUILD without it, and it seemed fine. However, I don't know if it was added intentionally because of some weird Inkscape plugins.