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

[miss-islington]

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.

(cherry picked from commit 46e6a28)

Co-authored-by: James Frost git@frost.cx

• Issue: http.server directories named index.html break directory listings #100474