typing_extensions: Drop Python 2.7, modernize build (python#931)

* Drop Python 2.7 support -- tests, linting
* Adopt pyproject.toml and a build backend (flit)
* Add Python 3.7+ types to README.rst

Author: AA-Turner

README.rst

@@ -50,7 +50,7 @@ This module currently contains the following:
 - ``Literal``
 - ``NewType``
 - ``NoReturn``
-- ``overload`` (note that older versions of ``typing`` only let you use ``overload`` in stubs)
+- ``overload``
 - ``OrderedDict``
 - ``ParamSpec``
 - ``ParamSpecArgs``
@@ -64,6 +64,14 @@ This module currently contains the following:
 - ``TypeGuard``
 - ``TYPE_CHECKING``
 
+Python 3.7+
+-----------
+
+- ``get_origin``
+- ``get_args``
+- ``get_type_hints``
+
+
 Other Notes and Limitations
 ===========================
 

pyproject.toml

@@ -0,0 +1,67 @@
+# Build system requirements.
+[build-system]
+requires = ["flit_core >=3.4,<4"]
+build-backend = "flit_core.buildapi"
+
+# Project metadata
+[project]
+name = "typing_extensions"
+version = "4.0.0-pre"
+description = "Backported and Experimental Type Hints for Python 3.6+"
+readme.text = """\
+Typing Extensions -- Backported and Experimental Type Hints for Python
+
+The ``typing`` module was added to the standard library in Python 3.5, but
+many new features have been added to the module since then.
+This means users of older Python versions who are unable to upgrade will not be
+able to take advantage of new types added to the ``typing`` module, such as
+``typing.Protocol`` or ``typing.TypedDict``.
+
+The ``typing_extensions`` module contains backports of these changes.
+Experimental types that may eventually be added to the ``typing``
+module are also included in ``typing_extensions``.
+"""
+readme.content-type = "text/x-rst"
+requires-python = ">=3.6"
+urls.Home = "https://github.com/python/typing/blob/master/typing_extensions/README.rst"
+license.file = "LICENSE"
+keywords = [
+    "annotations",
+    "backport",
+    "checker",
+    "checking",
+    "function",
+    "hinting",
+    "hints",
+    "type",
+    "typechecking",
+    "typehinting",
+    "typehints",
+    "typing"
+]
+# Classifiers list: https://pypi.org/classifiers/
+classifiers = [
+    "Development Status :: 3 - Alpha",
+    "Environment :: Console",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: Python Software Foundation License",
+    "Operating System :: OS Independent",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3 :: Only",
+    "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",
+    "Topic :: Software Development"
+]
+
+# Project metadata -- authors. Flit stores this as a list of dicts, so it can't
+# be inline above.
+[[project.authors]]
+name = "Guido van Rossum, Jukka Lehtosalo, Łukasz Langa, Michael Lee"
+email = "[email protected]"
+
+# This tells Flit that the module is stored in the src_py3 directory.
+[tool.flit.module]
+name = "src_py3/typing_extensions"