community/qt5-qtwebengine: build with python3 instead of python2
As per tsc#40 (closed), Alpine 3.16 should ideally ship without a Python 2 package. The only significant package remaining in main/ and community/ which requires python2 is qt5-qtwebengine. This commit makes qt5-qtwebengine build with python3 instead of python2 by adopting a (mostly upstreamed) patchset from Arch Linux.
This allows us to move python2 from community/ to testing/ soonish.
See #12740 (closed)