Refactor test_smtpserver.py using aiosmtpd

[jennifer-richards]

Describe the issue

For testing / dev environments, an test SMTP server is used to handle messages the Datatracker sends. This uses the smtpd library which was deprecated in Python 3.6 and dropped in Python 3.12. This is blocking #7929.

General instructions for migrating from amtpd to aiosmtpd are here: https://aiosmtpd.aio-libs.org/en/latest/migrating.html

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