Simplify determining if class type attribute is writable (PY-27866)

Don't consider the following attribute as writable in Python 3+:

```
    class A:
        attr = "attr"
        __slots__ = ("attr")

    A().attr
```

python/src/com/jetbrains/python/psi/types/PyClassTypeImpl.java

@@ -1,7 +1,6 @@
 // Copyright 2000-2017 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
 package com.jetbrains.python.psi.types;
 
-import com.google.common.collect.Iterables;
 import com.intellij.codeInsight.completion.CompletionUtil;
 import com.intellij.codeInsight.lookup.LookupElement;
 import com.intellij.codeInsight.lookup.LookupElementBuilder;
@@ -865,68 +864,14 @@ public class PyClassTypeImpl extends UserDataHolderBase implements PyClassType {
 
   @Override
   public boolean isAttributeWritable(@NotNull String name, @NotNull TypeEvalContext context) {
-    if (isDefinition() || PyUtil.isObjectClass(getPyClass())) return true;
+    final PyClass cls = getPyClass();
 
-    /*
-    The only difference between Py2 and Py3+ is that the following case is considered as writable in Py3+:
+    if (isDefinition() || PyUtil.isObjectClass(cls)) return true;
 
-    class A:
-        attr = "attr"
-        __slots__ = ("attr")
-
-    A().attr
-
-    Py3+ raises ValueError about conflict between __slots__ and class variable.
-    This case is handled in com.jetbrains.python.inspections.PyDunderSlotsInspection.Visitor.processSlot.
-    */
-    if (LanguageLevel.forElement(getPyClass()).isPython2()) {
-      return attributeIsWritableInPy2(name, context);
-    }
-    else {
-      return attributeIsWritableInPy3(name, context);
-    }
-  }
-
-  private boolean attributeIsWritableInPy2(@NotNull String name, @NotNull TypeEvalContext context) {
-    final List<String> slots = getPyClass().getSlots(context);
+    final List<String> slots = cls.getSlots(context);
     return slots == null ||
-           slots.contains(name) && getPyClass().findClassAttribute(name, true, context) == null ||
-           getPyClass().findProperty(name, true, context) != null;
-  }
-
-  private boolean attributeIsWritableInPy3(@NotNull String name, @NotNull TypeEvalContext context) {
-    boolean classAttrIsFound = false;
-    boolean slotIsFound = false;
-
-    for (PyClassLikeType type : Iterables.concat(Collections.singletonList(this), getAncestorTypes(context))) {
-      if (!(type instanceof PyClassType)) return true;
-
-      final PyClass cls = ((PyClassType)type).getPyClass();
-      if (PyUtil.isObjectClass(cls)) {
-        continue;
-      }
-
-      if (!cls.isNewStyleClass(context)) return true;
-
-      final List<String> ownSlots = cls.getOwnSlots();
-      if (ownSlots == null || ownSlots.contains(PyNames.DICT)) {
-        return true;
-      }
-
-      if (cls.findProperty(name, false, context) != null) {
-        return true;
-      }
-
-      if (!classAttrIsFound) {
-        classAttrIsFound = cls.findClassAttribute(name, false, context) != null;
-        if (ownSlots.contains(name)) {
-          if (classAttrIsFound) return true;
-          slotIsFound = true;
-        }
-      }
-    }
-
-    return slotIsFound && !classAttrIsFound;
+           slots.contains(name) && cls.findClassAttribute(name, true, context) == null ||
+           cls.findProperty(name, true, context) != null;
   }
 
   @Nullable

python/testData/inspections/PyDunderSlotsInspection/classAttrAssignmentAndOwnWithAttrAndInheritedSlotsPy3.py

@@ -9,5 +9,5 @@ C.attr = 'spam'
 print(C.attr)
 
 c = C()
-c.attr = 'spam'
+<warning descr="'C' object attribute 'attr' is read-only">c.attr</warning> = 'spam'
 print(c.attr)

python/testData/inspections/PyDunderSlotsInspection/classAttrAssignmentAndSlotsWithAttrPy3.py

@@ -6,5 +6,5 @@ Foo.attr = 'spam'
 print(Foo.attr)
 
 foo = Foo()
-foo.attr = 'spam'
+<warning descr="'Foo' object attribute 'attr' is read-only">foo.attr</warning> = 'spam'
 print(foo.attr)

python/testData/inspections/PyDunderSlotsInspection/inheritedClassAttrAssignmentAndOwnAndInheritedWithAttrSlotsPy3.py

@@ -9,5 +9,5 @@ C.attr = 'spam'
 print(C.attr)
 
 c = C()
-c.attr = 'spam'
+<warning descr="'C' object attribute 'attr' is read-only">c.attr</warning> = 'spam'
 print(c.attr)