python3.pkgs.markdown: fixup build

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

@@ -4,6 +4,9 @@
 , setuptools
 , nose
 , pyyaml
+, pythonOlder
+, importlib-metadata
+, isPy3k
 }:
 
 buildPythonPackage rec {
@@ -15,7 +18,13 @@ buildPythonPackage rec {
     sha256 = "1fafe3f1ecabfb514a5285fca634a53c1b32a81cb0feb154264d55bf2ff22c17";
   };
 
-  propagatedBuildInputs = [ setuptools ];
+  propagatedBuildInputs = [ 
+    setuptools 
+  ] ++ lib.optionals (pythonOlder "3.8") [
+    importlib-metadata
+  ];
+
+  disabled = !isPy3k;
 
   checkInputs = [ nose pyyaml ];