gunicorn: add 19.x branch for python2.7 support

The nixos/moinmoin module uses gunicorn, however the 20.0 version
dropped python2 support which broke the module as there's no python3
port planned for moinmoin: http://moinmo.in/Python3

pkgs/development/python-modules/gunicorn/19.nix

···
+{ stdenv, buildPythonPackage, fetchPypi
+, coverage
+, mock
+, pytest
+, pytestcov
+, setuptools
+}:
+
+buildPythonPackage rec {
+  pname = "gunicorn";
+  version = "19.10.0";
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "1080jk1ly8j0rc6lv8i33sj94rxjaskd1732cdq5chdqb3ij9ppr";
+  };
+
+  propagatedBuildInputs = [ setuptools ];
+
+  checkInputs = [ pytest mock pytestcov coverage ];
+
+  prePatch = ''
+    substituteInPlace requirements_test.txt --replace "==" ">=" \
+      --replace "coverage>=4.0,<4.4" "coverage"
+  '';
+
+  # better than no tests
+  checkPhase = ''
+    $out/bin/gunicorn --help > /dev/null
+  '';
+
+  pythonImportsCheck = [ "gunicorn" ];
+
+  meta = with stdenv.lib; {
+    homepage = "https://github.com/benoitc/gunicorn";
+    description = "WSGI HTTP Server for UNIX";
+    license = licenses.mit;
+  };
+}

pkgs/top-level/python-packages.nix

···
 
   rebulk = callPackage ../development/python-modules/rebulk { };
 
-  gunicorn = callPackage ../development/python-modules/gunicorn { };
+  gunicorn = if isPy27 then
+          callPackage ../development/python-modules/gunicorn/19.nix { }
+        else
+          callPackage ../development/python-modules/gunicorn { };
 
   hawkauthlib = callPackage ../development/python-modules/hawkauthlib { };