gh-100474: Fix handling of dirs named index.html in http.server (GH-100475)

If you had a directory called index.html or index.htm within a directory, it would cause http.server to return a 404 Not Found error instead of the directory listing. This came about due to not checking that the index was a regular file.

I have also added a test case for this situation.

Automerge-Triggered-By: GH:merwok

Lib/http/server.py

@@ -711,7 +711,7 @@ class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
                 return None
             for index in self.index_pages:
                 index = os.path.join(path, index)
-                if os.path.exists(index):
+                if os.path.isfile(index):
                     path = index
                     break
             else: