python36: 3.6.3 -> 3.6.4

pkgs/development/interpreters/python/cpython/3.6/default.nix

···
 
 let
   majorVersion = "3.6";
-  minorVersion = "3";
+  minorVersion = "4";
   minorVersionSuffix = "";
   pythonVersion = majorVersion;
   version = "${majorVersion}.${minorVersion}${minorVersionSuffix}";
···
 
   src = fetchurl {
     url = "https://www.python.org/ftp/python/${majorVersion}.${minorVersion}/Python-${version}.tar.xz";
-    sha256 = "1nl1raaagr4car787a2hmjv2dw6gqny53xfd6wisbgx4r5kxk9yd";
+    sha256 = "1fna7g8jxzl4kd2pqmmqhva5724c5m920x3fsrpsgskaylmr76qm";
   };
 
   NIX_LDFLAGS = optionalString stdenv.isLinux "-lgcc_s";