pythonPackages.dlib: move expression

Initially the expression was quite small (just inherited properties from
`pkgs.dlib`), but the more it grows the better it is to store it into
its own file.

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

@@ -0,0 +1,13 @@
+{ buildPythonPackage, dlib, python, pytest }:
+
+buildPythonPackage {
+  inherit (dlib) name src nativeBuildInputs buildInputs meta;
+
+  checkPhase = ''
+    ${python.interpreter} nix_run_setup test --no USE_AVX_INSTRUCTIONS
+  '';
+
+  patches = [ ./build-cores.patch ];
+
+  checkInputs = [ pytest ];
+}