python2.pkgs.sympy: keep 1.5.1 for python2 for sage

pkgs/development/python-modules/sympy/1_5.nix

@@ -0,0 +1,36 @@
+{ lib
+, buildPythonPackage
+, fetchPypi
+, fetchpatch
+, glibcLocales
+, mpmath
+}:
+
+buildPythonPackage rec {
+  pname = "sympy";
+  version = "1.5.1"; # Upgrades may break sage. Please test or ping @timokau.
+
+  src = fetchPypi {
+    inherit pname version;
+    sha256 = "d77901d748287d15281f5ffe5b0fef62dd38f357c2b827c44ff07f35695f4e7e";
+  };
+
+  checkInputs = [ glibcLocales ];
+
+  propagatedBuildInputs = [ mpmath ];
+
+  # tests take ~1h
+  doCheck = false;
+  pythonImportsCheck = [ "sympy" ];
+
+  preCheck = ''
+    export LANG="en_US.UTF-8"
+  '';
+
+  meta = with lib; {
+    description = "A Python library for symbolic mathematics";
+    homepage    = "https://www.sympy.org/";
+    license     = licenses.bsd3;
+    maintainers = with maintainers; [ lovek323 timokau ];
+  };
+}

pkgs/top-level/python-packages.nix

@@ -6299,7 +6299,10 @@ in {
     symengine = pkgs.symengine;
   };
 
-  sympy = callPackage ../development/python-modules/sympy { };
+  sympy = if isPy3k then
+    callPackage ../development/python-modules/sympy { }
+  else
+    callPackage ../development/python-modules/sympy/1_5.nix { };
 
   pilkit = callPackage ../development/python-modules/pilkit { };