In some environment, env doesn’t split arguments.
So your env is looking for python -u
in your path.
We can use sh to work around.
Replace your shebang with the following code lines and everything will be fine.
#!/bin/sh
''''exec python -u -- "$0" ${1+"$@"} # '''
# vi: syntax=python
p.s. we need not worry about the path to sh, right?