hdr: extract _BaseHeaderFooter

Extract _BaseHeaderFooter as a common base class for _Header and _Footer
to which common behaviors can be abstracted.

Author: scanny

docx/section.py

@@ -211,20 +211,20 @@ def top_margin(self, value):
         self._sectPr.top_margin = value
 
 
-class _Footer(BlockItemContainer):
-    """Page footer."""
+class _BaseHeaderFooter(BlockItemContainer):
+    """Base class for header and footer classes"""
 
     def __init__(self, sectPr, document_part):
         self._sectPr = sectPr
         self._document_part = document_part
 
 
-class _Header(BlockItemContainer):
-    """Page header."""
+class _Footer(_BaseHeaderFooter):
+    """Page footer."""
 
-    def __init__(self, sectPr, document_part):
-        self._sectPr = sectPr
-        self._document_part = document_part
+
+class _Header(_BaseHeaderFooter):
+    """Page header."""
 
     @property
     def is_linked_to_previous(self):