assert_smtp_socket fails with Python 3.13.0a3

Hello. In Fedora, we are trying to test our packages early with pre-releases of Python 3.13 to be prepared to upgrade to it in the development version of Fedora Linux 41. aiosmtpd fails a test.

To reproduce, I did this:

```diff
diff --git a/pytest.ini b/pytest.ini
index f2b9850..a681fff 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -8,9 +8,6 @@ addopts =
-ra
# show values of the local vars in errors:
--showlocals
- # coverage reports
- --cov=aiosmtpd/ --cov-report term
-asyncio_mode = auto
filterwarnings =
error
# TODO: Replace pkg_resources
diff --git a/tox.ini b/tox.ini
index 8282343..5e81227 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
[tox]
minversion = 3.9.0
-envlist = qa, static, docs, py{38,39,310,311,312,py3}-{nocov,cov,diffcov}
+envlist = qa, static, docs, py{38,39,310,311,312,313,py3}-{nocov,cov,diffcov}
skip_missing_interpreters = True

[testenv]
@@ -25,9 +25,9 @@ deps =
pytest >= 6.0 # Require >= 6.0 for pyproject.toml support (PEP 517)
pytest-mock
pytest-print
- pytest-profiling
pytest-sugar
py # needed for pytest-sugar as it doesn't declare dependency on it.
+ setuptools
!nocov: coverage>=7.0.1
!nocov: coverage[toml]
!nocov: coverage-conditional-plugin
```

The changes were necessary to even run the tests with Python 3.13. pytest-profiling is not ready for 3.13 and conftest imports pkg_resources.

Then I run:

```
$ tox -e py313-nocov
```

Or directly:

```
$ tox -e py313-nocov -- -k test_unixsocket
```

I got:

```
...
py313-nocov: commands[2]> pytest --verbose -p no:cov --tb=short -k test_unixsocket
============================= test session starts ==============================
platform linux -- Python 3.13.0a3, pytest-8.0.0, pluggy-1.4.0 -- .../aiosmtpd/.tox/py313-nocov/bin/python
cachedir: .tox/py313-nocov/.pytest_cache
rootdir: .../aiosmtpd
configfile: pytest.ini
testpaths: aiosmtpd/tests/, aiosmtpd/qa/
plugins: sugar-1.0.0, print-1.0.0, mock-3.12.0
collecting ... collected 565 items / 564 deselected / 1 selected

aiosmtpd/tests/test_server.py::TestUnthreaded::test_unixsocket FAILED [100%]

=================================== FAILURES ===================================
________________________ TestUnthreaded.test_unixsocket ________________________
aiosmtpd/tests/test_server.py:454: in test_unixsocket
assert_smtp_socket(cont)
autostop_loop = <_UnixSelectorEventLoop running=False closed=False debug=True>
cont =
runner = .starter at 0x7f91d0602f20>
safe_socket_dir = PosixPath('/tmp/tmpdoje2ko7')
self =
sockfile = PosixPath('/tmp/tmpdoje2ko7/smtp')
aiosmtpd/tests/test_server.py:104: in assert_smtp_socket
assert Path(controller.unix_socket).exists()
E AssertionError: assert False
E + where False = ()
E + where = PosixPath('/tmp/tmpdoje2ko7/smtp').exists
E + where PosixPath('/tmp/tmpdoje2ko7/smtp') = Path('/tmp/tmpdoje2ko7/smtp')
E + where '/tmp/tmpdoje2ko7/smtp' = .unix_socket
controller =
------------------------------ Captured log call -------------------------------
INFO mail.log:smtp.py:408 Available AUTH mechanisms: LOGIN(builtin) PLAIN(builtin)
INFO mail.log:smtp.py:521 Peer: ''
INFO mail.log:smtp.py:613 '' handling connection
DEBUG mail.log:smtp.py:597 '' << b'220 carbon Python SMTP 1.4.4.post2'
DEBUG mail.log:smtp.py:287 _handle_client readline: b'EHLO socket.test\r\n'
INFO mail.log:smtp.py:287 '' >> b'EHLO socket.test'
DEBUG mail.log:smtp.py:597 '' << b'250-carbon'
DEBUG mail.log:smtp.py:597 '' << b'250-SIZE 33554432'
DEBUG mail.log:smtp.py:597 '' << b'250-8BITMIME'
DEBUG mail.log:smtp.py:597 '' << b'250-SMTPUTF8'
DEBUG mail.log:smtp.py:597 '' << b'250 HELP'
DEBUG mail.log:smtp.py:287 _handle_client readline: b'QUIT\r\n'
INFO mail.log:smtp.py:287 '' >> b'QUIT'
DEBUG mail.log:smtp.py:597 '' << b'221 Bye'
INFO mail.log:smtp.py:528 '' connection lost
INFO mail.log:smtp.py:759 '' Connection lost during _handle_client()
============================= slowest 10 durations =============================
3.31s call aiosmtpd/tests/test_server.py::TestUnthreaded::test_unixsocket
0.00s setup aiosmtpd/tests/test_server.py::TestUnthreaded::test_unixsocket
0.00s teardown aiosmtpd/tests/test_server.py::TestUnthreaded::test_unixsocket
=========================== short test summary info ============================
FAILED aiosmtpd/tests/test_server.py::TestUnthreaded::test_unixsocket - Asser...
====================== 1 failed, 564 deselected in 3.45s =======================
py313-nocov: exit 1 (3.70 seconds) .../aiosmtpd> pytest --verbose -p no:cov --tb=short -k test_unixsocket pid=856635
.pkg-cpython313: _exit> python /usr/lib/python3.12/site-packages/pyproject_api/_backend.py True setuptools.build_meta
py313-nocov: FAIL code 1 (5.21=setup[0.60]+cmd[0.04,0.88,3.70] seconds)
evaluation failed :( (5.39 seconds)
```

This is with:

```
$ python3.13 --version
Python 3.13.0a3

$ .tox/py313-nocov/bin/pip list
Package Version
-------------- -----------
aiosmtpd 1.4.4.post2
atpublic 4.0
attrs 23.2.0
bandit 1.7.7
colorama 0.4.6
iniconfig 2.0.0
markdown-it-py 3.0.0
mdurl 0.1.2
packaging 23.2
pbr 6.0.0
pip 23.2.1
pluggy 1.4.0
py 1.11.0
Pygments 2.17.2
pytest 8.0.0
pytest-mock 3.12.0
pytest-print 1.0.0
pytest-sugar 1.0.0
PyYAML 6.0.1
rich 13.7.0
setuptools 69.0.3
stevedore 5.1.0
termcolor 2.4.0
```