Revert default version to '3'

[zooba]

While we are in initial beta, the default tag is 3.14-dev, to encourage testing of prereleases (on the assumption that only prerelease testers will have prerelease PyManager):

pymanager/src/manage/commands.py

Line 25 in 0f75607

DEFAULT_TAG = "3.14-dev"

Before we make a stable release (which presumably will be just after 3.14.0 releases), we should switch this to just '3'.

[zooba]

This is now partly done in #146, making the default launch tag 3 and the default install tag 3-dev.

We'll change the default install tag to just 3 after 3.14 stable is released.