Eli Schwartz authored Dec 13, 2021 and Werner Lemberg committed Dec 17, 2021

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.