tools: make update-dbus-docs compatible with Python 3.6

668b3a42fe 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.

tools/update-dbus-docs.py

@@ -197,7 +197,7 @@ def subst_output(document, programlisting, stats):
         print(f'COMMAND: {shlex_join(argv)}')
 
     try:
-        out = subprocess.check_output(argv, text=True)
+        out = subprocess.check_output(argv, universal_newlines=True)
     except FileNotFoundError:
         print(f'{executable} not found, ignoring', file=sys.stderr)
         return