The previous change to check the return code of `run_command` invocations
caused the CI to fail. Although most scripts used `python_exe` as the
program command, the script to determine the project version did not.
But, all scripts used `python` as the shebang, and this is not available on
all systems. Particularly Debian does not provide a `python` command,
though `python3` does exist. This meant that formerly the version number
was lacking, and now the build simply fails.
Instead, rely on `python3` since it is guaranteed to exist when running
meson, and `python2` is end of life anyway.
57fd61805f