Make PyStatementPart interface extend newer PyStatementListContainer

Both interfaces provide the same method getStatementList

python/psi-api/src/com/jetbrains/python/psi/PyElsePart.java

@@ -15,17 +15,10 @@
  */
 package com.jetbrains.python.psi;
 
-import org.jetbrains.annotations.Nullable;
-
 /**
  * The 'else:' part of various compound statements.
  * User: dcheryasov
  * Date: Mar 15, 2009 9:34:51 PM
  */
 public interface PyElsePart extends PyStatementPart {
-  /**
-   * @return the body of the 'else' part.
-   */
-  @Nullable
-  PyStatementList getStatementList();
 }

python/psi-api/src/com/jetbrains/python/psi/PyStatementPart.java

@@ -15,19 +15,19 @@
  */
 package com.jetbrains.python.psi;
 
-import org.jetbrains.annotations.Nullable;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * Abstract part of a multipart statement.
  * User: dcheryasov
  * Date: Mar 16, 2009 4:34:59 AM
  */
-public interface PyStatementPart extends PyElement {
+public interface PyStatementPart extends PyElement, PyStatementListContainer {
   PyStatementPart[] EMPTY_ARRAY = new PyStatementPart[0];
 
   /**
    * @return the body of the part.
    */
-  @Nullable
+  @NotNull
   PyStatementList getStatementList();
 }