recipes: Don't try to use `python3` on Windows

Python 3 on Windows is still called python.exe; changing it to use
python3.exe breaks Windows builds.

For harfbuzz and libsoup, the proper fix is to port the recipes to
Meson. The Meson ports are already upstream.
7 jobs for fix-python3-windows in 98 minutes and 15 seconds (queued for 2 seconds)