PY-20057 Unify type vars constrained by class types and instance types

so that annotations like "def f(x: Type[T]) -> T" (and vice-versa) work
as expected.

Additionally, I've added a new method toClass() in PyInstantiableType
interface that is supposed to be the opposite to existing toInstance().

python/pluginJava/com/jetbrains/python/psi/impl/PyJavaClassType.java

@@ -128,11 +128,18 @@ public class PyJavaClassType implements PyClassLikeType {
     return myDefinition;
   }
 
+  @NotNull
   @Override
   public PyClassLikeType toInstance() {
     return myDefinition ? new PyJavaClassType(myClass, false) : this;
   }
 
+  @NotNull
+  @Override
+  public PyClassLikeType toClass() {
+    return myDefinition ? this : new PyJavaClassType(myClass, true);
+  }
+
   @Nullable
   @Override
   public String getClassQName() {