bootstrap.sh does not detect Python 3.8 on macOS, neither the one from macOS itself nor from MacPorts

After running:
`$ ./bootstrap.sh --prefix=$HOME --with-python=/opt/local/bin/python3.8`
still Python 2.7 of macOS is detected. Tried with and without path.
Expected would be that it takes the mentioned Python version from this binary.
A workaround is to use
`$ sudo port select python python38`
This makes Python 3.8 the default reachable with just `python`, and `./bootstrap.sh` works. But this is most likely not what the author of this script intended.