The re stdlib module requires attrs that don't exist in the backported 3.4 version.
Technically, we are changing our install behavior beyond what is necessary. Previously, enum34 was used for 3.4 and 3.5 as well, and it happened not to conflict, but I think it's better to use the latest bug-fixed stdlib versions as long as they meet the needs of `cryptography`, which is what depends on enum34. That way, at least the various stdlib modules are guaranteed not to conflict with each other.
The primary motivation is to avoid a branch, giving bugs one fewer place to hide. But, as a bonus, more people get a more bugfixed version of argparse. (To use the example from the argparse docs, people stuck on Python 3.2.3 can get bugfixes that made it into the stdlib only in 3.2.4.)
