fix attachment unicodeError

[criptocoko]

This is a fix to handle emails with attachments that have characters that are not in range(128), in this case we encode them in base64.

[iredmail]

Hi @criptocoko,

As requested in #757, could you please show us code snippet to reproduce the issue?

[iredmail]

Hi @cclauss, could you help check the black --check . issue in GitHub Actions result?
I ran the latest black version locally but it's all fine, not sure why it failed in GitHub Actions.

$ black --check .
All done! ✨ 🍰 ✨
29 files would be left unchanged.

[iredmail]

By the way, this change looks good to me after checked source code of py encoders.encode_base64().