Fix tox for Python >= 3.12

Fix import error:
```
py312-cov: commands[2]> pytest --cov --cov-report=xml --cov-report=html --cov-report=term --tb=short
ImportError while loading conftest '/home/rominf/dev/aiosmtpd/aiosmtpd/tests/conftest.py'.
aiosmtpd/tests/conftest.py:15: in <module>
    from pkg_resources import resource_filename
E   ModuleNotFoundError: No module named 'pkg_resources'
```
by installing `setuptools`.

tox.ini

@@ -28,6 +28,7 @@ deps =
     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