www/node: build with python < 3.7

r535334 introduced a patch for tools/genv8constants.py that only worked
with Python >= 3.7. The "text" argument added to Python's
subprocess.Popen() is an alias for "universal_newlines", which works on
all versions of Python, including 2.7.

PR: 248167
Submitted by: James French <james@french.id.au>
Reported by: Miroslav Lachman <000.fbsd@quip.cz>
MFH: 2020Q3
Sponsored by: Miles AS