bpo-29750: support non-ASCII passwords in smtplib

[Windsooon]

issue: https://bugs.python.org/issue29750

I updated the PR based on the review from #8938

https://bugs.python.org/issue29750

[Windsooon]

Please review this PR when you have time @taleinat :D

[ZackerySpytz]

You don't need to create a new PR in order to address review comments. Just update the original PR.

[Windsooon]

Hello, @ZackerySpytz , I use a wrong branch name at the very beginning then deleted it locally by mistake. That is why I use a new PR here.

[seblu]

Python 3.8.0 is still impacted by this.

Patched version 3.8.0 accept passwords with utf8 chars.

[Leon-001]

Hi,
I just wanted to give some motivation: I had a special character in my password and it took some time until I have figured out, what was going on. It would be great, if this patch could be reviewed!
Thank you all!

[max-toepfer]

Hello, lovely Python dev folks,
as another affected individual I wanted to ask, if this topic is still alive?
Thank you so much for the contributions!

[MaxwellDupre]

Let smtplib support non-ASCII passwords, better
smtplib support for UTF-8 passwords

[arhadthedev]

Closing in favor of SASLprep-based gh-103611.