python,python3: Fix overridden usr/bin symlinks

python,python3: Fix overridden usr/bin symlinks

Currently, all files in usr/bin (presumably all Python scripts) are run
through sed to replace the shebang; sed will overwrite the file whether
or not a match is found. This causes symlinks to be overridden and made
into copies of their targets. python[3]-base and python[3]-dev are
affected by this.

This adds the --follow-symlinks flag to sed, in addition to using
$(SED), so that symlinks are not overridden.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>