diff --git a/pkgs/development/python-modules/cssselect2/default.nix b/pkgs/development/python-modules/cssselect2/default.nix
index 381c114b44e..45a9920dbe2 100644
--- a/pkgs/development/python-modules/cssselect2/default.nix
+++ b/pkgs/development/python-modules/cssselect2/default.nix
@@ -1,4 +1,4 @@
-{ lib, buildPythonPackage, fetchPypi, tinycss2, pytest, pytestrunner, pytestcov, pytest-flake8, pytest-isort, glibcLocales }:
+{ lib, buildPythonPackage, fetchPypi, tinycss2, pytest, pytestrunner }:
 
 buildPythonPackage rec {
   pname = "cssselect2";
@@ -9,11 +9,21 @@ buildPythonPackage rec {
     sha256 = "505d2ce3d3a1d390ddb52f7d0864b7efeb115a5b852a91861b498b92424503ab";
   };
 
+  # We're not interested in code quality tests
+  postPatch = ''
+    substituteInPlace setup.py \
+      --replace "pytest-cov" "" \
+      --replace "pytest-flake8" "" \
+      --replace "pytest-isort" ""
+    substituteInPlace setup.cfg \
+      --replace "--cov=cssselect2" "" \
+      --replace "--flake8" "" \
+      --replace "--isort" ""
+  '';
+
   propagatedBuildInputs = [ tinycss2 ];
 
-  checkInputs = [ pytest pytestrunner pytestcov pytest-flake8 pytest-isort glibcLocales ];
-
-  LC_ALL = "en_US.UTF-8";
+  checkInputs = [ pytest pytestrunner ];
 
   meta = with lib; {
     description = "CSS selectors for Python ElementTree";
diff --git a/pkgs/development/python-modules/isort/default.nix b/pkgs/development/python-modules/isort/default.nix
index 5a7e50d4332..bb9958a6fef 100644
--- a/pkgs/development/python-modules/isort/default.nix
+++ b/pkgs/development/python-modules/isort/default.nix
@@ -1,22 +1,24 @@
-{ lib, buildPythonPackage, fetchPypi, isPy27, futures, mock, pytest }:
+{ lib, buildPythonPackage, fetchPypi, isPy27, futures, backports_functools_lru_cache, mock, pytest }:
 
-buildPythonPackage rec {
+let
+  skipTests = lib.optional isPy27 "test_standard_library_deprecates_user_issue_778";
+  testOpts = lib.concatMapStringsSep " " (t: "--deselect test_isort.py::${t}") skipTests;
+in buildPythonPackage rec {
   pname = "isort";
-  version = "4.3.4";
+  version = "4.3.16"; # Note 4.x is the last version that supports Python2
 
   src = fetchPypi {
     inherit pname version;
-    sha256 = "1y0yfv56cqyh9wyg7kxxv9y5wmfgcq18n7a49mp7xmzka2bhxi5r";
+    sha256 = "1v6lapqhc33rxr9698lqjyb49fis27i42p3ymngrw95py3qf7y08";
   };
 
-  propagatedBuildInputs = lib.optional isPy27 futures;
+  propagatedBuildInputs = lib.optionals isPy27 [ futures backports_functools_lru_cache ];
 
   checkInputs = [ mock pytest ];
 
+  # isort excludes paths that contain /build/, so test fixtures don't work with TMPDIR=/build/
   checkPhase = ''
-    py.test test_isort.py -k "not test_long_line_comments \
-                          and not test_import_case_produces_inconsistent_results_issue_472 \
-                          and not test_no_extra_lines_issue_557"
+    PATH=$out/bin:$PATH TMPDIR=/tmp/ pytest ${testOpts}
   '';
 
   meta = with lib; {