Unconditionally depend on jupyter_sphinx for dev (qiskit-community#608)

The original constraint was added in qiskit-communitygh-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.

(cherry picked from commit 0e3716c)

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