Change python from a run to a build dep of www/node

Change python from a run to a build dep of www/node. The current version of
node doesn't install any Python files.

PR: 210309
Reviewed by: Bradley T. Hughes (Maintainer)
Approved by: pi (ports)
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6699