pythonPackages.contextvars: relax constraint on immutables version (#55660)

The build was broken before since we have immutables 0.9

pkgs/development/python-modules/contextvars/default.nix

@@ -10,6 +10,8 @@ buildPythonPackage rec {
     sha256 = "09fnni8cyxm070bfv9ay030qbyk0dfds5nq77s0p38h33hp08h93";
   };
 
+  # pull request for this patch is https://github.com/MagicStack/contextvars/pull/9
+  patches = [ ./immutables_version.patch ];
   propagatedBuildInputs = [ immutables ];
 
   meta = {

pkgs/development/python-modules/contextvars/immutables_version.patch

@@ -0,0 +1,11 @@
+--- ./setup.py	2018-07-30 15:40:14.000000000 +0000
++++ ./setup.py	2019-02-12 18:33:24.984430303 +0000
+@@ -17,7 +17,7 @@
+     packages=['contextvars'],
+     provides=['contextvars'],
+     install_requires=[
+-        'immutables==0.6',
++        'immutables>=0.6',
+     ],
+     license='Apache License, Version 2.0',
+     classifiers=[