For instance on Debian based host OS 'python' is not provided by
default, which results is following error when trying to execute
oe-stylize.py script:

  /usr/bin/env: ‘python’: No such file or directory

Update the shebang to explicitly reference 'python3' instead of
'python', which should make the script better out-of-the-box
compatible with larger variety of host OSes on which Yocto based
development work takes place.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>