doku2git: Modify Change to be an ABC

Engineering aside, this further reduces warnings from PyCharm.

doku2git.py

@@ -5,6 +5,7 @@
 from typing import List, Dict, Iterable, Callable, Generator
 from operator import attrgetter
 
+import abc
 import os.path
 import os
 import collections
@@ -57,7 +58,7 @@ class User(object):
 
 
 # -----------------------------------------------------------------------------
-class Change(object):
+class Change(abc.ABC):
     """
     The base class for all change records in the Dokuwiki instance.
 
@@ -88,6 +89,25 @@ class Change(object):
     def __str__(self):
         return f'{self.user}@{self.timestamp}:{self.page} "{self.description}"'
 
+    @abc.abstractmethod
+    def _update(self, _src, _dst: str) -> Generator[str, None, None]:
+        """
+        Yield the commands required to unpack this change.
+        :param _src: The Dokuwiki instance this change belongs to.
+        :param _dst: The absolute path to the root of the git repository
+        :return:
+        """
+        pass
+
+    @abc.abstractmethod
+    def _attic(self, _src) -> str:
+        """
+        Find the path to the history data within the Dokuwiki directories.
+        :param _src: The Dokuwiki instance this change belongs to.
+        :return: The relative path from the root of Dokuwiki to the data.
+        """
+        pass
+
     def _delete(self, _src: str, _dst: str) -> Generator[str, None, None]:
         """
         Generates the required commands to delete a given page
@@ -243,7 +263,7 @@ class MediaChange(Change):
             f'{basename}.{self.timestamp}{extension}'
         )
 
-    def _update(self, src: str, dst: str) -> Generator[str, None, None]:
+    def _update(self, src, dst: str) -> Generator[str, None, None]:
         """
         Yields the commands required to unpack this one change.
         :param src: The Dokuwiki instance this change belongs to.