Release 0.4.0

This release was of course actually done a long time ago,
but I am redoing it in the new repo.
diff --git a/setup.py b/setup.py
index 95bf467..392970c 100644
--- a/setup.py
+++ b/setup.py
@@ -2,7 +2,7 @@
 
 from setuptools import setup
 
-version = '0.4.0-dev'
+version = '0.4.0'
 description = 'Experimental type system extensions for programs checked with the mypy typechecker.'
 long_description = '''
 Mypy Extensions