[FIX] base: compatible with python 3.9

Partial backport of 4f03a5f1 for 13.0

Some methods were removed in python 3.9, use the alias instead

* `base64.encodestring` -> `base64.encodebytes`
* `base64.decodestring` -> `base64.decodebytes`

Fixes odoo/odoo#80652

Part-of: odoo/odoo#80999