PY-21692 Fixed: Return type of sum(List[Decimal]) is 'Any' (should be 'Decimal')

Add skeletons for builtin sum function

python/testSrc/com/jetbrains/python/Py3TypeTest.java

@@ -475,6 +475,12 @@ public class Py3TypeTest extends PyTestCase {
            "expr = max(1, 2, 3)");
   }
 
+  // PY-21692
+  public void testSumResult() {
+    doTest("int",
+           "expr = sum([1, 2, 3])");
+  }
+
   private void doTest(final String expectedType, final String text) {
     myFixture.configureByText(PythonFileType.INSTANCE, text);
     final PyExpression expr = myFixture.findElementByText("expr", PyExpression.class);

python/testSrc/com/jetbrains/python/PyTypeTest.java

@@ -1524,6 +1524,12 @@ public class PyTypeTest extends PyTestCase {
            "expr = max(1, 2, 3)");
   }
 
+  // PY-21692
+  public void testSumResult() {
+    doTest("int",
+           "expr = sum([1, 2, 3])");
+  }
+
   private static List<TypeEvalContext> getTypeEvalContexts(@NotNull PyExpression element) {
     return ImmutableList.of(TypeEvalContext.codeAnalysis(element.getProject(), element.getContainingFile()).withTracing(),
                             TypeEvalContext.userInitiated(element.getProject(), element.getContainingFile()).withTracing());