www/node: use python 3 as build dependency

Do not depend on the deprecated python 2.7 package to build Node.js,
patching tools/genv8constants.py to continue to work and generate the
same output with python 3 that it did with python 2.

While here, add all *.py scripts used by the build to SHEBANG_FILES,
which causes the installed lldb_commands.py script to change, hence the
PORTREVISION bump.

PR: 246036
Reported by: lumiwa@gmail.com
Sponsored by: Miles AS