Move mypy_extensions version to 0.3.0 (about to release) (#3676)

setup.py

diff --git a/setup.py b/setup.py
index 59d634d..b0ffbc5 100644
--- a/setup.py
+++ b/setup.py
@@ -4,7 +4,7 @@
 
 from distutils.core import setup
 
-version = '0.3.0-dev'
+version = '0.3.0'
 description = 'Experimental type system extensions for programs checked with the mypy typechecker.'
 long_description = '''
 Mypy Extensions