tools: make update-dbus-docs compatible with Python 3.6
668b3a42 allowed update-dbus-docs.py to start
running on Cent OS 8 (instead of skipping). But subprocess.check_output()'s
text argument didn't exist until Python 3.7 and C8 is still running
Python 3.6. Use universal_newlines instead for backwards compatibility.
(cherry picked from commit 934d0d02)