Skip "data:" URIs in ImageConverter

sphinx/transforms/post_transforms/images.py

@@ -200,6 +200,9 @@ def match(self, node: nodes.image) -> bool:
         elif set(self.guess_mimetypes(node)) & set(self.app.builder.supported_image_types):
             # builder supports the image; no need to convert
             return False
+        elif node['uri'].startswith('data:'):
+            # all data URI MIME types are assumed to be supported
+            return False
         elif self.available is None:
             # store the value to the class variable to share it during the build
             self.__class__.available = self.is_available()