feat(users/Profpatsch): add python3Lib writer

Smol writer to create a python lib directly from a nix string.
The resulting library can be consumed by the writePython3 writer.

Change-Id: Id3d793564d230b38a08f65140bda4287285e1a72
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2310
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/Profpatsch/writers/tests.nix

@@ -0,0 +1,22 @@
+{ depot, pkgs, python3Lib }:
+
+let
+  testLib = python3Lib {
+    name = "test_lib";
+  } ''
+    def test():
+      return "test"
+  '';
+
+  pythonWithLib = pkgs.writers.writePython3 "python-with-lib" {
+    libraries = [ testLib ];
+  } ''
+    import test_lib
+
+    assert(test_lib.test() == "test")
+  '';
+
+in {
+  inherit
+    pythonWithLib;
+}