Use Semantic Versioning for typing_extensions (#907)

Bump version to 4.0.0-pre

Author: srittau

CHANGELOG

@@ -0,0 +1,4 @@
+# Changes in version 4.0.0
+
+Starting with version 4.0.0, typing_extensions uses Semantic Versioning.
+See the README for more information.

MANIFEST.in

@@ -1,3 +1,3 @@
-include LICENSE README.rst
+include CHANGELOG LICENSE README.rst
 include src_py3/typing_extensions.py
 include src_py3/test_typing_extensions.py

README.rst

@@ -19,6 +19,13 @@ the ``typing`` module from PyPi instead of using this one unless
 specifically writing code that must be compatible with multiple Python
 versions or requires experimental types.
 
+Starting with version 4.0.0, ``typing_extensions`` uses
+`Semantic Versioning <https://semver.org/>`_. The
+major version is incremented for all backwards-incompatible changes, including
+dropping support for older Python versions. Therefore, it's safe to depend
+on ``typing_extensions`` like this: ``typing_extensions >=x.y, <(x+1)``,
+where ``x.y`` is the first version that includes all features you need.
+
 Included items
 ==============
 

setup.cfg

@@ -1,4 +1,5 @@
 [metadata]
+version = 4.0.0-pre
 license-file = LICENSE
 
 [options]

setup.py

@@ -9,7 +9,6 @@
                      'to install typing_extensions.\n')
     exit(1)
 
-version = '3.10.0.2'
 description = 'Backported and Experimental Type Hints for Python 3.6+'
 long_description = '''\
 Typing Extensions -- Backported and Experimental Type Hints for Python
@@ -40,7 +39,6 @@
 ]
 
 setup(name='typing_extensions',
-      version=version,
       description=description,
       long_description=long_description,
       author='Guido van Rossum, Jukka Lehtosalo, Łukasz Langa, Michael Lee',