build: Remove deprecated python3 Meson module use

We are using the deprecated python3 module for historical reasons. We
used to support both Python 2 and Python 3 in the Autotools build, but
since we switched to Meson we *know* we have Python 3 available. This
allows us to use a shebang line in the Python scripts we use in the
build, instead of invoking them through the Python interpreter in a
custom target; Meson knows how to handle shebangs portably as well.

This change removes the only deprecation warning coming from Meson when
configuring the Pango build.
4 jobs for ebassi/for-master in 1 minute and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #918756
linux-fedora

00:00:41

passed #918757
win32-ps
msys2-mingw64

00:01:27

 
  Analysis
passed #918758
asan
asan-build

00:00:46

 
  Docs
passed #918759
reference

00:00:57