Python: Add integration test verifying NIX_PYTHONPATH with Mypy

pkgs/development/interpreters/python/tests/test_nix_pythonprefix/default.nix

@@ -0,0 +1,25 @@
+{ interpreter, writeText, runCommandNoCC }:
+
+let
+
+  python = let
+    packageOverrides = self: super: {
+      typeddep = super.callPackage ./typeddep {};
+    };
+  in interpreter.override {inherit packageOverrides; self = python;};
+
+  pythonEnv = python.withPackages(ps: [
+    ps.typeddep
+    ps.mypy
+  ]);
+
+  pythonScript = writeText "myscript.py" ''
+    from typeddep import util
+    s: str = util.echo("hello")
+    print(s)
+  '';
+
+in runCommandNoCC "${interpreter.name}-site-prefix-mypy-test" {} ''
+  ${pythonEnv}/bin/mypy ${pythonScript}
+  touch $out
+''