Epub: Add WebP mime type

When building docs with WebP images using the EPUB builder results in this warning:

`WARNING: unknown mimetype for _images/<image_name>.webp, ignoring`

This commit fixes this by adding the mapping for webp-files to the correct mime type.

sphinx/builders/_epub_base.py

@@ -56,6 +56,7 @@
     '.xhtml': 'application/xhtml+xml',
     '.css': 'text/css',
     '.png': 'image/png',
+    '.webp': 'image/webp',
     '.gif': 'image/gif',
     '.svg': 'image/svg+xml',
     '.jpg': 'image/jpeg',