kernel-devsrc: fix incorrect python shebang replacement

kernel-devsrc: fix incorrect python shebang replacement

Update the sed replacement rule to strictly match '/usr/bin/python'
(with no trailing characters)

The previous sed rule was too broad and could incorrectly change Python
shebangs such as in
/lib/modules/6.16.11-yocto-standard/build/scripts/macro_checker.py from
'#!/usr/bin/python3' to '#!/usr/bin/env python33'.

Signed-off-by: Jiaying Song <jiaying.song.cn@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>