Delay UID change after loop start

[nim-odoo]

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What do these changes do?

Listening to a port < 1024 without --nosetuid leads to a permission error.

The UID change is done too early: we should first open the port, then change the UID.

Are there changes in behavior for the user?

Users can now drop privileges while listening to port < 1024.

Related issue number

Fixes https://github.com/aio-libs/aiosmtpd/issues/304

Checklist

• [x] I think the code is well written
• [ ] Unit tests for the changes exist
• [ ] tox testenvs have been executed in the following environments:
  • [ ] Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-{nocov,cov,diffcov}, qa, docs
  • [ ] Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • [ ] WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • [ ] FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • [ ] Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• [ ] Documentation reflects the changes
• [x] Add a news fragment into the NEWS.rst file
  • Add under the "aiosmtpd-next" section, creating one if necessary
    • You may create subsections to group the changes, if you like
  • Use full sentences with correct case and punctuation
  • Refer to relevant Issue if applicable

[waynew]

Note that there is a QA issue with comment line being a little too long.

[nim-odoo]

Hello @pepoluan , is there a chance this can be looked at? Thanks

[pepoluan]

Hello @pepoluan , is there a chance this can be looked at? Thanks

I'll see what I can do this weekend.