Possible standards violation: SMTPUTF8 and 8BITMIME

Was reading RFC 6531 and came upon [Section 3.1](https://tools.ietf.org/html/rfc6531#section-3.1) which explicitly states:

> 8. Servers offering this extension MUST provide support for, and announce, the 8BITMIME extension [[RFC6152]](https://tools.ietf.org/html/rfc6152).

Now if I initialize `SMTP` with `enable_SMTPUTF8=True` _and_ `decode_data=True`, then the `EHLO` session returns:

```python
b'220 MY-LAPTOP Python SMTP 1.2.3a2\r\n'
b'250-MY-LAPTOP\r\n'
b'250-SIZE 33554432\r\n'
b'250-SMTPUTF8\r\n'
b'250 HELP\r\n'
```

which is in violation of RFC 6531 § 3.1 number 8 above.

There's also a bit of complexity w.r.t. mail body itself (i.e., data following the `DATA` command); [RFC 6531 § 3.6](https://tools.ietf.org/html/rfc6531#section-3.6) specifies that

> ... The SMTPUTF8 extension MAY be used as follows (assuming it is appropriate given the body content):
>
> - with the BODY=8BITMIME parameter [RFC6152], or
>
> - with the BODY=BINARYMIME parameter, if the SMTP server advertises BINARYMIME [RFC3030].

This will require some rethinking on the logic behind `enable_SMTPUTF8` and `decode_data`