Update Hypothesis and attrs for compatibility with newer nixpkgs

Newer typing in nixpkgs requires newer Hypothesis than it has.

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

@@ -5,6 +5,21 @@ 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.pytest.overrideAttrs (old: {
+        doInstallCheck = false;
+      });
+
       # new tahoe-lafs dependency
       eliot = python-super.callPackage ./eliot.nix { };
       # new autobahn requires a newer cryptography