I'm sure there's plenty of uglier stuff on far bigger scales. But good design is all in the small details.

Looking at this one small syntactic difference explains a lot about why C++ is as ugly as it is. It has to pack in a lot of complexity everywhere, even at the most basic level, such as how to label a lambda expression. The ugliness may simply be a cost of offering much more power (simplicity is difficult/impossible when you need to support complex base abstractions). But it's still ugly. And therefore a big reason why it's not very Pythonic as the OP said.

Most of the ugliness there is coming from using "__"-prefixed identifiers (reserved for implementations) and macros hiding code specific to certain platforms or versions of the standard.

Even if those weren't directly caused by flaws in C++, a find/replace to remove "__"s and the five conditional macros does little to make this code less complex.