PR: Show changelog after updates (Update manager)

[ccordoba12]

Description of Changes

• This will let users know the most important changes after each release.
• For now it'll only work with our standalone installers.
• Add Help > Show changelog menu entry in case users want to check the changelog again.
• Add markdown-it-py as a new dependency to render the changelog's markdown to html (although it was already a transitive one).

Issue(s) Resolved

Fixes #

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @ccordoba12

[ccordoba12]

@dalthviz, please check if this logic is correct (by manually testing the installer from this PR) in the following scenarios:

• We shouldn't show the changelog when Spyder is started for the first time (i.e. with clean settings) nor the next time after that.
• If Spyder doesn't have clean settings (i.e. it was started before, you can simulate that by clearing our settings and starting the 6.1.2 installer), then we should show the changelog when the installer from this branch is launched.
• We shouldn't show the changelog the next time Spyder is started after the previous scenario.

[dalthviz]

@ccordoba12 gave a check to the cases above, locally on Windows using the installer from this PR, and seems to me that things are working as expected 👍

However, one thing I noticed (at least on my Windows setup) is that the changelog shows a scrollbar which doesn't follow the theme style. Shouldn't the scrollbar follow a dark style (when using the dark theme) like for example with the Help or Online Help panes:

[ccordoba12]

gave a check to the cases above, locally on Windows using the installer from this PR, and seems to me that things are working as expected 👍

Great!

However, one thing I noticed (at least on my Windows setup) is that the changelog shows a scrollbar which doesn't follow the theme style.

Good catch!

Shouldn't the scrollbar follow a dark style (when using the dark theme) like for example with the Help or Online Help panes

I'll ask Andres to address that after merging this PR.

[ccordoba12]

@dalthviz, please approve this PR so I can merge it if you agree with it.

[ccordoba12]

@meeseeksdev please backport to 6.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout 6.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 ae8a41bac5926e769b9a9b1be064d79c3abbe6e6

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #25647: PR: Show changelog after updates (Update manager)'

4. Push to a named branch:

git push YOURFORK 6.x:auto-backport-of-pr-25647-on-6.x

5. Create a PR against branch 6.x, I would have named this PR:

"Backport PR #25647 on branch 6.x (PR: Show changelog after updates (Update manager))"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.