python.pkgs.rawkit: add patch for python37 compatibility

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

@@ -1,4 +1,4 @@
-{ stdenv, fetchPypi, buildPythonPackage
+{ stdenv, fetchPypi, buildPythonPackage, fetchpatch
 , libraw
 , pytest, mock }:
 
@@ -11,6 +11,14 @@ buildPythonPackage rec {
     sha256 = "0vrhrpr70i61y5q5ysk341x1539ff1q1k82g59zq69lv16s0f76s";
   };
 
+  patches = [
+    # Python 3.7 compatibility
+    (fetchpatch {
+      url = https://github.com/photoshell/rawkit/commit/663e90afa835d398aedd782c87b8cd0bff64bc9f.patch;
+      sha256 = "1cdw0x9bgk0b5jnpjnmd8jpbaryarr3cjqizq44366qh3l0jycxy";
+    })
+  ];
+
   buildInputs = [ libraw ];
 
   checkInputs = [ pytest mock ];