Original was really rough because the core team had gone in the wrong direction on migration, and the Python io module was hell as well.
By 3.3-3.4 it was relatively smooth sailing but that took a lot of work from the community and core team both (reminder that Python 2.7 was released after 3.1, backporting a number of features to make compatibility easier, and old features were reintroduced as late as 3.5).