Merge 0.8-dev to master

[soininen]

Plan to merge 0.8-dev to master:

• Ensure CHANGELOG.md is up-to-date (Check that CHANGELOG.md is up-to-date for 0.8-dev #2693, Check that CHANGELOG.md is up-to-date for 0.8-dev Spine-Database-API#386)
• Show Upgrade notification window on Toolbox startup to users in master (just once) (Get Upgrade notification ready for the 0.8-dev -> master merge #2699)
• Release hopefully last 0.7.x to PyPI (Update CHANGELOG.md for 0.7.3, 0.7.4 #2694)
• Give a short grace period until 2024-04-29
• Make release branches for 0.7.x in all four repositories, update requirements.txt to pull from other release branches
  - [ ] Revert master back to last-merged-to-entity-branch tag so as to minimize merge conflicts in all four repositories
• Merge 0.8-dev to master in all four repositories, resolve merge conflicts (see @PekkaSavolainen's post below)
• Make sure requirements.txt points again to master, not 0.8-dev
• Make sure the Upgrade notification is gone from 0.8
  - [ ] Close eyes, hope for the best We are not descended from fearful men.
• Release 0.8 to PyPI (remember to update CHANGELOG.md)
• Update Bundle generation (Automate bundle generation #2720)
• Update Downloads page (Update generate_readme.py to check artifacts from 'master' branch Downloads#1)

Edit: dropped the "Revert master" step according to the discussion below.

[ptsavol]

I tested merging in my fork repos. I merged the latest commit from 0.8-dev to master (just like we merge feature branches), then solved all conflicts based on what was in 0-8-dev branch. There were only about 10 files in spine-toolbox and spine-items that needed conflict resolution, less than 10 files in both spine-engine and spinedb-api. Then I just fixed the requirements.txt in both spine-toolbox and spine-items to point to master again, fixed some repo paths in github actions and voilá. All tests passed on the first try and the app seems to be working fine. There was some traceback related to the cat animation but I'm not sure if this has something to do with a brand new PySide6 version (6.7.0). On second try I could not reproduce the traceback anymore. If you want to try master after 0.8-dev has been merged, clone Spine-Toolbox from my personal stash and do pip install -r requirements.txt in a conda or venv.

[soininen]

I tested merging in my fork repos.

@PekkaSavolainen Thank you so much for the test! Sounds like we can omit the revert step and just merge as usual.

[soininen]

Merging PR #2701 ticks the second box in the task list.

[soininen]

#2725 needs to be fixed before we can proceed with this. I am working on it.

[soininen]

I will fix #2723 before releasing 0.8 to PyPI.

[soininen]

With the release of 0.8.0 we are done here.