Pin nbconvert to < 7.14 to workaround issue (backport #119)

[mergify]

This is an automatic backport of pull request #119 done by Mergify.
Cherry-pick of 10a69bb has failed:

On branch mergify/bp/stable/0.2/pr-119
Your branch is up to date with 'origin/stable/0.2'.

You are currently cherry-picking commit 10a69bb.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   constraints.txt

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.com

[woodsp-ibm]

I fixed the conflict. Stable constraints still has the numpy pin in it unlike on main.

[coveralls]

Pull Request Test Coverage Report for Build 7492788845

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.038%

---

Totals
Change from base Build 6882242094:	0.0%
Covered Lines:	6444
Relevant Lines:	7157

---

💛 - Coveralls