Possible standards violation: 8BITMIME (decode_data=True) implies Strict ASCII (if SMTPUTF8 not enabled)

Currently, the _only_ way to make `SMTP` return the `8BITMIME` capability is by setting `decode_data=True`

However, this activated the piece of code in `smtp_DATA` which enforces "Strict ASCII".

Indicating `8BITMIME` but rejecting "arbitrary octet-aligned data" is a violation of [RFC 6152](https://tools.ietf.org/html/rfc6152), notably Section 3.

I propose:

1. We separate `decode_data` with "Strict ASCII", and
2. For `decode_data` _without_ "Strict ASCII", the encoding used for `.decode()` should be "Latin-1"

Point No. 1 above means that this change have the potential of breaking things; so I propose we bump up the version to **1.3** while we're at it.