Addressed comments and added changelog entry

CHANGES

@@ -15,6 +15,8 @@ Features added
 
 * #10840: One can cross-reference including an option value: ``:option:`--module=foobar```.
   Patch by Martin Liska.
+* #10614: Fixed a number of bugs in inheritance diagrams that resulted in
+  missing or broken links
 
 Bugs fixed
 ----------

sphinx/ext/inheritance_diagram.py

@@ -30,7 +30,7 @@ class E(B): pass
 
 import builtins
 import inspect
-import pathlib
+import posixpath
 import re
 from importlib import import_module
 from typing import Any, Dict, Iterable, List, Optional, Tuple, cast
@@ -402,21 +402,21 @@ def html_visit_inheritance_diagram(self: HTMLTranslator, node: inheritance_diagr
     # Create a mapping from fully-qualified class names to URLs.
     graphviz_output_format = self.builder.env.config.graphviz_output_format.upper()
     current_filename = self.builder.current_docname + self.builder.out_suffix
-    current_dir = pathlib.PurePath(current_filename).parent
+    current_dir = posixpath.dirname(current_filename)
     urls = {}
     pending_xrefs = cast(Iterable[addnodes.pending_xref], node)
     for child in pending_xrefs:
         if child.get('refuri') is not None:
             # Construct the name from the URI if the reference is external via intersphinx
-            if not child.get('internal', True):
-                refname = child['refuri'].rsplit('#', 1)[-1]
-            else:
+            if child.get('internal', True):
                 refname = child['reftitle']
+            else:
+                refname = child['refuri'].rsplit('#', 1)[-1]
 
             # For SVG output, relative URIs need to be re-pathed to where the SVG file will be
             if graphviz_output_format == 'SVG' and '://' not in child['refuri']:
                 # URI relative to src dir (typically equivalent to stripping all leading ../)
-                uri_rel_to_srcdir = (current_dir / child['refuri']).as_posix()
+                uri_rel_to_srcdir = posixpath.join(current_dir, child['refuri'])
                 # URI relative to image dir (typically equivalent to prepending ../)
                 uri_rel_to_imagedir = relpath(uri_rel_to_srcdir, self.builder.imagedir)
                 urls[refname] = canon_path(uri_rel_to_imagedir)
@@ -426,8 +426,8 @@ def html_visit_inheritance_diagram(self: HTMLTranslator, node: inheritance_diagr
             if graphviz_output_format == 'SVG':
                 # URI relative to image dir (typically equivalent to prepending ../)
                 uri_rel_to_imagedir = relpath(current_filename, self.builder.imagedir)
-                urls[child['reftitle']] = canon_path(uri_rel_to_imagedir) +\
-                                          '#' + child.get('refid')
+                urls[child['reftitle']] = (canon_path(uri_rel_to_imagedir) +
+                                           '#' + child.get('refid'))
             else:
                 urls[child['reftitle']] = '#' + child.get('refid')