[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 33961ff6f (in the 3.2 branch, though, Python 3.3
is supported).

src/modules/bootloader/main.py

@@ -471,7 +471,7 @@ def efi_boot_next():
     """
     boot_mgr = libcalamares.job.configuration["efiBootMgr"]
     boot_entry = None
-    efi_bootvars = subprocess.check_output([boot_mgr], text=True)
+    efi_bootvars = subprocess.check_output([boot_mgr], universal_newlines=True)
     for line in efi_bootvars.split('\n'):
         if not line:
             continue