Pin Sphinx to <8.2 temporarily to avoid nbsphinx problem (#1514)

The currently latest release of nbsphinx (0.9.6) is not compatible with
Sphinx 8.2.0, so here Sphinx is pinned to `<8.2`. See:

spatialaudio/nbsphinx#825

The issue is that nbsphinx does `import sphinx` and relies `sphinx` to
import all of its submodules which it no longer does.

Note -- qiskit-experiments mainly uses `jupyter-sphinx` now but it still
uses the `nbgallery` directive for the landing pages for the tutorials,
so it can not simply be dropped as a dependency (maybe the gallery could
be formatted without it).

Closes
#1513

Author: wshanks

requirements-dev.txt

@@ -26,7 +26,7 @@ nbsphinx
 pylatexenc
 qiskit-sphinx-theme
 reno>=4.1.0
-sphinx>=6.2.1
+sphinx>=6.2.1,<8.2
 sphinx-copybutton
 sphinx-design
 sphinx-remove-toctrees