PY-21040 Fixed: False positive: os.popen(...).close() doesn't return anything

Update os.popen skeleton to return class which has return value in close()

python/testSrc/com/jetbrains/python/inspections/Py3NoneFunctionAssignmentInspectionTest.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2000-2016 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.jetbrains.python.inspections;
+
+import com.intellij.testFramework.LightProjectDescriptor;
+import com.jetbrains.python.fixtures.PyTestCase;
+import org.jetbrains.annotations.Nullable;
+
+public class Py3NoneFunctionAssignmentInspectionTest extends PyTestCase {
+
+  @Nullable
+  @Override
+  protected LightProjectDescriptor getProjectDescriptor() {
+    return ourPy3Descriptor;
+  }
+
+  // PY-21040
+  public void testOsPopenClose() {
+    doMultiFileTest();
+  }
+
+  private void doMultiFileTest() {
+    myFixture.copyDirectoryToProject("inspections/PyNoneFunctionAssignmentInspection/" + getTestName(false) + "/", "");
+    myFixture.configureByFile("a.py");
+    myFixture.enableInspections(PyNoneFunctionAssignmentInspection.class);
+    myFixture.checkHighlighting(false, false, true);
+  }
+}