Create mypy_extensions package. (#2228)

Closes #2210. See also #2206.

README.md

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+Mypy Extensions
+===============
+
+The "mypy_extensions" module defines experimental extensions to the
+standard "typing" module that are supported by the mypy typechecker.
+

mypy_extensions.py

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+"""Defines experimental extensions to the standard "typing" module that are
+supported by the mypy typechecker.
+
+Example usage:
+    from mypy_extensions import TypedDict
+"""
+
+# NOTE: This module must support Python 2.7 in addition to Python 3.x
+
+# (TODO: Declare TypedDict and other extensions here)

setup.py

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+#!/usr/bin/env python
+
+# NOTE: This package must support Python 2.7 in addition to Python 3.x
+
+from distutils.core import setup
+
+version = '0.1-dev'
+description = 'Experimental type system extensions for programs checked with the mypy typechecker.'
+long_description = '''
+Mypy Extensions
+===============
+
+The "mypy_extensions" module defines experimental extensions to the
+standard "typing" module that are supported by the mypy typechecker.
+'''.lstrip()
+
+classifiers = [
+    'Development Status :: 2 - Pre-Alpha',
+    'Environment :: Console',
+    'Intended Audience :: Developers',
+    'License :: OSI Approved :: MIT License',
+    'Operating System :: POSIX',
+    'Programming Language :: Python :: 2',
+    'Programming Language :: Python :: 2.7',
+    'Programming Language :: Python :: 3',
+    'Programming Language :: Python :: 3.3',
+    'Programming Language :: Python :: 3.4',
+    'Programming Language :: Python :: 3.5',
+    'Topic :: Software Development',
+]
+
+setup(
+    name='mypy_extensions',
+    version=version,
+    description=description,
+    long_description=long_description,
+    author='David Foster',
+    author_email='david@dafoster.net',
+    url='http://www.mypy-lang.org/',
+    license='MIT License',
+    platforms=['POSIX'],
+    py_modules=['mypy_extensions'],
+    classifiers=classifiers,
+)