[REF] mail: remove deprecated email formataddr

[PEP 594] is going to depreciate the legacy `email.message.Message` API
and its related modules. Among them the `email.utils.formataddr`
function, this function takes a `(name, email)` pair and returns a
string value suitable for From, To and Cc headers.

The stdlib function is capable of handling several character encoding
and two binary-to-ascii encoding. Odoo only uses `utf-8` which is
compatible with the base64 b2a encoding. The re-implementation of that
function has been simplified to only support base64-ed utf-8 and ascii.

[PEP 594]: https://www.python.org/dev/peps/pep-0594/\#email-lagacy-api