s/setup.cfg/pyproject.toml/ in the development docs (#3058)

Co-authored-by: Alexander Clausen <alex@gc-web.de>
Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

docs/development.rst

@@ -94,12 +94,11 @@ Code style guide
 
 - First and foremost, the linters configured for the project must pass; this generally means following PEP-8 rules,
   as codified by: ``flake8``, ``black``, ``isort``, ``pyupgrade``.
-- The supported Python versions (and the code syntax to use) are listed in the ``setup.cfg`` file
-  in the ``options/python_requires`` entry. However, there are some files that have to be kept compatible
+- The supported Python versions (and the code syntax to use) are listed in the ``pyproject.toml`` file
+  in the ``project/requires-python`` entry. However, there are some files that have to be kept compatible
   with Python 2.7 to allow and test for running Python 2 envs from tox. They are listed in ``.pre-commit-config.yaml``
   under ``repo: https://github.com/asottile/pyupgrade`` under ``hooks/exclude``.
   Please do not attempt to modernize them to Python 3.x.
-- Packaging options should be specified within ``setup.cfg``; ``setup.py`` is only kept for editable installs.
 - All code (tests too) must be type annotated as much as required by ``mypy``.
 - We use a line length of 120.
 - Exception messages should only be capitalized (and ended with a period/exclamation mark) if they are multi-sentenced,