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