QuTiP master transitioning to v5

[Simon Cross]

Greetings,

For the last few years we've been developing QuTiP 5 in the dev.major
branch. We're now close to releasing the first alpha version of QuTiP
5 and it's time to think about how the branches should be used during
the life of QuTiP 5 and beyond.

It seems sensible to have the master branch be the "main development
branch" for QuTiP, which implies merging dev.major into master.
However, that could be disruptive for those who rely on the master
branch to be QuTiP 4.X, so we plan to proceed as follows:

From today (15 December):

- fixes to QuTiP 4.7.X should go to the qutip-4.7.X branch
- the master branch won't receive any new fixes.

From 15 January:

- we'll merge the dev.major branch into master
- new QuTiP 5 development will happen in master
- the dev.major branch won't receive any new fixes.

If anyone will be massively inconvenienced by this plan, please let us
know and we'll see if we can accommodate.

At your service,
Simon