Use a list for epub_exclude_files. Fixes a build failure.
--- a/Doc/conf.py
+++ b/Doc/conf.py
@@ -440,7 +440,7 @@
 
 epub_author = 'Python Documentation Authors'
 epub_publisher = 'Python Software Foundation'
-epub_exclude_files = ('index.xhtml', 'download.xhtml')
+epub_exclude_files = ['index.xhtml', 'download.xhtml']
 
 # index pages are not valid xhtml
 # https://github.com/sphinx-doc/sphinx/issues/12359