Unconditionally depend on jupyter_sphinx for dev (#608)

The original constraint was added in gh-505, at a time when Aer was
specifically requiring jupyter_sphinx in its own `requirements-dev.txt`
under certain constraints.  The constraint no longer appears to be
needed, so all it serves to do now is to cause the docs build to fail on
Python 3.9.

requirements-dev.txt

@@ -8,8 +8,7 @@ sphinx-tabs>=1.1.11
 sphinx-autodoc-typehints
 stestr>=2.5.0
 ddt>=1.2.0,!=1.4.0
-jupyter;python_version<'3.9'
-jupyter-sphinx;python_version<'3.9'
+jupyter-sphinx
 reno
 matplotlib
 pyfakefs