Move metadata to pyproject + drop Python 3.7

MANIFEST.in

@@ -1 +1,6 @@
-include LICENSE
+# files necessary for testing sdist
+graft tests
+include tox.ini
+
+global-exclude *.py[cod]
+global-exclude .DS_Store

pyproject.toml

@@ -0,0 +1,34 @@
+[build-system]
+requires      = ["setuptools>=75.0"]
+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"
+authors     = [
+    {name = "The mypy developers", email = "jukka.lehtosalo@iki.fi"}
+]
+license     = {text = "MIT"}
+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",
+    "Programming Language :: Python :: 3.12",
+    "Topic :: Software Development",
+]
+requires-python = ">=3.7"
+
+[project.urls]
+Homepage = "https://github.com/python/mypy_extensions"
+
+[tool.setuptools]
+py-modules = ["mypy_extensions"]