downgrade typing instead of upgrading a bunch of stuff

attrs.nix

-{ lib, stdenv, buildPythonPackage, fetchPypi, pytest, hypothesis, zope_interface
-, pympler, coverage, six, clang }:
-
-buildPythonPackage rec {
-  pname = "attrs";
-  version = "19.2.0";
-
-  src = fetchPypi {
-    inherit pname version;
-    sha256 = "sha256:15hkbvwzw87474ra22mmzbbikkqkrnk9npifa1pw7lv32qp4j4zr";
-  };
-
-  # macOS needs clang for testing
-  checkInputs = [
-    pytest hypothesis zope_interface pympler coverage six
-  ] ++ lib.optionals (stdenv.isDarwin) [ clang ];
-
-  checkPhase = ''
-    py.test
-  '';
-
-  # To prevent infinite recursion with pytest
-  doCheck = false;
-
-  meta = with lib; {
-    description = "Python attributes without boilerplate";
-    homepage = https://github.com/hynek/attrs;
-    license = licenses.mit;
-  };
-}

hypothesis.nix

-{ lib, buildPythonPackage, fetchFromGitHub
-, isPy3k, attrs, coverage, enum34
-, doCheck ? true, pytest, pytest_xdist, flaky, mock, pexpect
-}:
-buildPythonPackage rec {
-  # https://hypothesis.readthedocs.org/en/latest/packaging.html
-
-  # Hypothesis has optional dependencies on the following libraries
-  # pytz fake_factory django numpy pytest
-  # If you need these, you can just add them to your environment.
-
-  version = "4.38.3";
-  pname = "hypothesis";
-
-  # Use github tarballs that includes tests
-  src = fetchFromGitHub {
-    owner = "HypothesisWorks";
-    repo = "hypothesis-python";
-    rev = "hypothesis-python-${version}";
-    sha256 = "1w9mjc3bad8qgx3pygawvvwrbdrm4yix0ys4x6ry6hhln85xnpxx";
-  };
-
-  postUnpack = "sourceRoot=$sourceRoot/hypothesis-python";
-
-  propagatedBuildInputs = [ attrs coverage ] ++ lib.optional (!isPy3k) [ enum34 ];
-
-  checkInputs = [ pytest pytest_xdist flaky mock pexpect ];
-  inherit doCheck;
-
-  checkPhase = ''
-    rm tox.ini # This file changes how py.test runs and breaks it
-    py.test tests/cover
-  '';
-
-  meta = with lib; {
-    description = "A Python library for property based testing";
-    homepage = https://github.com/HypothesisWorks/hypothesis;
-    license = licenses.mpl20;
-  };
-}

overlays.nix

@@ -5,19 +5,16 @@ self: super: {
 
   python27 = super.python27.override {
     packageOverrides = python-self: python-super: {
-      # # A newer version of Hypothesis is required for compatibility with the
-      # # typing module which gets pulled in by some dependency or other.
-      # #
-      # # A bug report filed against nixpkgs -
-      # # https://github.com/NixOS/nixpkgs/issues/70431
-      # hypothesis = python-super.callPackage ./hypothesis.nix { };
-
-      # # The newer hypothesis requires a newer attrs.
-      # attrs = python-super.callPackage ./attrs.nix { };
-
-      # # The newer hypothesis or attrs breaks the pytest test suite.
-      # pytest = python-super.callPackage ./pytest.nix { };
-
+      # The newest typing is incompatible with the packaged version of
+      # Hypothesis.  Upgrading Hypothesis is like pulling on a loose thread in
+      # a sweater.  I pulled it as far as pytest where I found there was no
+      # upgrade route because pytest has dropped Python 2 support.
+      # Fortunately, downgrading typing ends up being fairly straightforward.
+      #
+      # For now.  This is, no doubt, a sign of things to come for the Python 2
+      # ecosystem - the early stages of a slow, painful death by the thousand
+      # cuts of incompatibilities between libraries with no maintained Python
+      # 2 support.
       typing = python-super.callPackage ./typing.nix { };
 
       # new tahoe-lafs dependency

pytest.nix

-{ stdenv, buildPythonPackage, pythonOlder, fetchPypi, attrs, hypothesis, py
-, setuptools_scm, setuptools, six, pluggy, funcsigs, isPy3k, more-itertools
-, atomicwrites, mock, writeText, pathlib2, wcwidth, packaging, isPyPy, python
-}:
-buildPythonPackage rec {
-  version = "5.2.1";
-  pname = "pytest";
-
-  disabled = !isPy3k;
-
-  preCheck = ''
-    # don't test bash builtins
-    rm testing/test_argcomplete.py
-  '';
-
-  src = fetchPypi {
-    inherit pname version;
-    sha256 = "3805d095f1ea279b9870c3eeae5dddf8a81b10952c8835cd628cf1875b0ef032";
-  };
-
-  checkInputs = [ hypothesis mock ];
-  nativeBuildInputs = [ setuptools_scm ];
-  propagatedBuildInputs = [ attrs py setuptools six pluggy more-itertools atomicwrites wcwidth packaging ]
-    ++ stdenv.lib.optionals (pythonOlder "3.6") [ pathlib2 ];
-
-  doCheck = !isPyPy; # https://github.com/pytest-dev/pytest/issues/3460
-
-  # Ignored file https://github.com/pytest-dev/pytest/pull/5605#issuecomment-522243929
-  checkPhase = ''
-    runHook preCheck
-    $out/bin/py.test -x testing/ -k "not test_collect_pyargs_with_testpaths" --ignore=testing/test_junitxml.py
-    runHook postCheck
-  '';
-
-  # Remove .pytest_cache when using py.test in a Nix build
-  setupHook = writeText "pytest-hook" ''
-    pytestcachePhase() {
-        find $out -name .pytest_cache -type d -exec rm -rf {} +
-    }
-    preDistPhases+=" pytestcachePhase"
-  '';
-
-  pythonImportsCheck = [
-    "pytest"
-  ];
-
-  meta = with stdenv.lib; {
-    homepage = https://docs.pytest.org;
-    description = "Framework for writing tests";
-    maintainers = with maintainers; [ domenkozar lovek323 madjar lsix ];
-    license = licenses.mit;
-  };
-}