fix: Disallow nbsphinx v0.8.8 to avoid empty "raw" directive bug

[matthewfeickert]

Description

Disallow nbsphinx v0.8.8 as it introduces a bug for empty 'raw' directives (c.f. spatialaudio/nbsphinx#620). This is fixed in spatialaudio/nbsphinx#621 and so should work its way into the next nbsphinx release. 👍

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Disallow nbsphinx v0.8.8 as it introduces a bug for empty 'raw' directives.
   - c.f. https://github.com/spatialaudio/nbsphinx/issues/620

[codecov]

Codecov Report

Merging #1742 (4d99705) into master (af6c09a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1742   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4270     4270           
  Branches      683      683           
=======================================
  Hits         4190     4190           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.25% <ø> (ø)
doctest	60.58% <ø> (ø)
unittests	96.18% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update af6c09a...4d99705. Read the comment docs.