Replace PYTHONSHEBANG with valid shebang
    
    Replace the @PYTHONSHEBANG@ substitution with a valid #!/usr/bin/python3
    shebang. This turns Python .in files into valid Python files. The files
    can now be checked with pylint and IDEs recognize the files as Python
    files.
    
    The shebang is still replaced with "#!$(PYTHON) -E" to support
    platform-python.
    
    Related: https://pagure.io/freeipa/issue/7984
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>
    Reviewed-By: Francois Cami <fcami@redhat.com>