[bootloader] Repair Python 3.6 compatibility

Argument *text* is an addition in 3.7, while the Calamares 3.3
branch supports Python 3.6 and later. Use the 'backwards compatibility'
name of the parameter, *universal_newlines*.

Cherry-picked from 33961ff6 (in the 3.2 branch, though, Python 3.3
is supported).