Some of both (for example, % formatting got added back to bytes objects in 3.5). Also, supporting 2.5 (or even 2.4) together with Python 3 was particularly inc

Some of both (for example, % formatting got added back to bytes objects in 3.5). Also, supporting 2.5 (or even 2.4) together with Python 3 was particularly inconvenient, so things sped up after people stopped caring about those (which took a long time because some people were running extended-support enterprise distributions).