Bump immutables dependency

0.6 has a crash fix.

Author: Fuyukai

setup.py

@@ -17,7 +17,7 @@
     packages=['contextvars'],
     provides=['contextvars'],
     install_requires=[
-        'immutables==0.5',
+        'immutables==0.6',
     ],
     license='Apache License, Version 2.0',
     classifiers=[