Update trove classifiers and python_requires to reflect supported Python versions

[browniebroke]

No description provided.

[jacekbj]

The future belongs to pyproject.toml. I guess it's still worth updating the existing code first.

[browniebroke]

While I agree, it might need a bit more discussion before doing it. One could be tempted to use Poetry, hatch or PDM when doing so... I leave it up to the maintainers to voice their preferences.

[browniebroke]

Example of things that need to be reworked if we change to pyproject.toml: the test suite calls setup.py directly atm:

django-datadog-logger/Makefile

Lines 53 to 54 in c90df5b

	test: ## run tests quickly with the default Python
	DJANGO_SETTINGS_MODULE=tests.settings python setup.py test