Move setuptools config from setup.cfg with pyproject.toml

Supported since setuptools 61.0.0, more standard.

pyproject.toml

@@ -1,3 +1,32 @@
 [build-system]
-requires = ["setuptools"]
+requires = ["setuptools>=61.0.0"]
 build-backend = "setuptools.build_meta"
+
+[project]
+name = "mypy_extensions"
+version = "1.0.0-dev"
+description = "Type system extensions for programs checked with the mypy type checker."
+readme = "README.md"
+license = {file = "LICENSE"}
+requires-python = ">=3.7"
+authors = [
+    {name = "The mypy developers", email = "[email protected]"},
+]
+classifiers = [
+    "Development Status :: 5 - Production/Stable",
+    "Environment :: Console",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.7",
+    "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
+    "Topic :: Software Development",
+]
+[project.urls]
+Repository = "https://github.com/python/mypy_extensions"
+
+[tool.setuptools]
+py-modules = ["mypy_extensions"]