Modernize packaging a bit

Adding pyproject.toml allows the package to be installed using PEP517
instead of legacy `setup.py install` method. At least pip says so in a
warning when installing without the `wheel` package available.

Change setup.py to setup.cfg (using a script[0]) because setup.py is out
of fashion these days.

Remove MANIFEST.ini since LICENSE is implied[1].

[0] https://github.com/gvalkov/setuptools-py2cfg
[1] https://setuptools.pypa.io/en/latest/references/keywords.html#keyword-license-files

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"

setup.cfg

@@ -1,2 +1,32 @@
-[bdist_wheel]
-universal=0
+[metadata]
+name = mypy_extensions
+version = 1.0.0-dev
+author = The mypy developers
+author_email = jukka.lehtosalo@iki.fi
+license = MIT License
+description = Type system extensions for programs checked with the mypy type checker.
+url = https://github.com/python/mypy_extensions
+long_description = Mypy Extensions
+    ===============
+
+    The "mypy_extensions" module defines extensions to the standard "typing" module
+    that are supported by the mypy type checker and the mypyc compiler.
+
+classifiers =
+    Development Status :: 5 - Production/Stable
+    Environment :: Console
+    Intended Audience :: Developers
+    License :: OSI Approved :: MIT License
+    Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.5
+    Programming Language :: Python :: 3.6
+    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
+
+[options]
+py_modules = mypy_extensions
+python_requires = >=3.5