python3Packages.selectors2: fix mapping import

The library is otherwise unmaintained and should be removed once
something larger comes up.

pkgs/development/python-modules/selectors2/default.nix

@@ -10,6 +10,10 @@ buildPythonPackage rec {
     sha256 = "1f1bbaac203a23fbc851dc1b5a6e92c50698cc8cefa5873eb5b89eef53d1d82b";
   };
 
+  patches = [
+    ./mapping-import.patch
+  ];
+
   checkInputs = [ nose psutil mock ];
 
   checkPhase = ''

pkgs/development/python-modules/selectors2/mapping-import.patch

@@ -0,0 +1,14 @@
+diff --git a/selectors2.py b/selectors2.py
+index 1625a30..c4a1231 100644
+--- a/selectors2.py
++++ b/selectors2.py
+@@ -22,7 +22,8 @@
+ # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ # SOFTWARE.
+ 
+-from collections import namedtuple, Mapping
++from collections import namedtuple
++from collections.abc import Mapping
+ import errno
+ import math
+ import platform