Bump mypy_extensions version to 0.4.0-dev (i.e. post-release-0.3.0) (#4418)

setup.py

diff --git a/setup.py b/setup.py
index b4918f4..417b3fd 100644
--- a/setup.py
+++ b/setup.py
@@ -2,7 +2,7 @@
 
 from setuptools import setup
 
-version = '0.3.0'
+version = '0.4.0-dev'
 description = 'Experimental type system extensions for programs checked with the mypy typechecker.'
 long_description = '''
 Mypy Extensions