I suspect that many Pythoners will install Python3 (with homebrew) or other on OSX (and whatever other platforms still come with just Python2)
If you maintain many FOSS libs like I do, together with tox it's priceless.
What if I want to distribute a package to an audience that doesn't have a strong Python background.
What would otherwise be a simple 'pip install' just turned into a complex multi-step process to install a new version, update PATH, and use virtualenv to avoid conflicts with the default install used internally by the OS.
These all seem like 'simple' fixes to a regular Python user but they set a huge barrier-of-entry for others.
'Just use [insert python specific fix]' is a UX failure for a language that emphasizes usability.
To reiterate. Python3 is ready for regular python users/devs. Not so much for the rest who consume python applications.