Revert "python3Packages.pip-tools: 6.3.1 -> 6.4.0"

This reverts commit 68f6e0321f2325a06ad25b3c7ab8b11b7691331a.

pkgs/development/python-modules/pip-tools/default.nix

···
 
 buildPythonPackage rec {
   pname = "pip-tools";
-  version = "6.4.0";
+  version = "6.3.1";
 
   disabled = pythonOlder "3.6";
 
   src = fetchPypi {
     inherit pname version;
-    sha256 = "65553a15b1ba34be5e43889345062e38fb9b219ffa23b084ca0d4c4039b6f53b";
+    sha256 = "992d968df6f1a19d4d37c53b68b3d4b601b894fb3ee0926d1fa762ebc7c7e9e9";
   };
 
   checkInputs = [