Fix _FakeServer to immediately close client connections

[cuu508]

What do these changes do?

This implements _FakeServer.connection_made() which immediately closes client connections.

Without this, if the SMTP server throws an exception during initialization, Controller.stop() gets stuck indefinitely waiting on active connections.

Note: this only happens in Python 3.12+. Earlier versions of Python allowed wait_closed() to complete regardless of active connections.

With this change the TestFactory testcases can again be enabled for Python 3.12+.

Are there changes in behavior for the user?

No change for the happy path, as _FakeServer is only used if the SMTP server throws exception during initialization.

Related issue number

Fixes: #394

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 24.04): {py312}
  • 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
• Add a news fragment into the NEWS.rst file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (f40ac96) to head (6d59b9d).
Report is 38 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   97.86%   97.75%   -0.11%     
==========================================
  Files          23       23              
  Lines        5707     5698       -9     
  Branches      764      763       -1     
==========================================
- Hits         5585     5570      -15     
- Misses         76       83       +7     
+ Partials       46       45       -1     

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