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
+''

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

@@ -0,0 +1,11 @@
+{ buildPythonPackage }:
+
+
+buildPythonPackage {
+
+  pname = "typeddep";
+  version = "1.3.3.7";
+
+  src = ./.;
+
+}

pkgs/development/interpreters/python/tests/test_nix_pythonprefix/typeddep/setup.py

@@ -0,0 +1,18 @@
+from setuptools import setup
+
+setup(**{
+    'name': 'typeddep',
+    'version': '1.3.3.7',
+    'description': 'Minimal repro to test mypy and site prefixes with Nix',
+    'long_description': None,
+    'author': 'adisbladis',
+    'author_email': 'adisbladis@gmail.com',
+    'maintainer': None,
+    'maintainer_email': None,
+    'url': None,
+    'packages': ['typeddep'],
+    'package_data': {'': ['*']},
+    'install_requires': [],
+    'entry_points': {},
+    'python_requires': '>=3.7,<4.0',
+})

pkgs/development/interpreters/python/tests/test_nix_pythonprefix/typeddep/typeddep/util.py

@@ -0,0 +1,2 @@
+def echo(s: str) -> str:
+    return s