Since all our python scripts have a proper python3 shebang, there is no benefit
to letting meson autodetect them. On linux, meson will just uses exec(), so the
shebang is used anyway. The only difference should be in how meson reports the
script and that the detection won't fail for (most likely misconfigured)
non-UTF8 locales.
Closes#5855.
This is useful on systems like NixOS, where python3 is not in
/usr/bin/python3 as well as for people using alternative ways to
install python such as virtualenv/pyenv.
