community/py3-pyclipper: use clipper library from Alpine package

To save installation space, and to use the exact same implementation of polygon clipping across the system.

Also taking the source code from pypi instead of github, like most other Alpine py3 packages. Other than github, pypi is putting the git release tag into the zip bundles, which is then used by Python setuptools. When setuptools is installing a package, it is checking for the version tag, and the installation fails with a warning when the version tag cannot be inferred. Before this change, the APKBUILD would run git init to work around this setuptools check. After this change, that hack is not needed anymore.