Pin sphinx version temporarily (Qiskit#4556)

Sphinx released 3.1.0 yesterday which has introduced 1821 warnings to
the terra docs builds. Until these can all be fixed the CI jobs are
failing because we enforce no warnings as sphinx warnings almost always
point to real docs bugs. This commit pins the sphinx version we use for
testing and local builds until we can fix all the newly introduced
warnings.

requirements-dev.txt

@@ -17,7 +17,7 @@ pylatexenc>=1.4
 ddt>=1.2.0,!=1.4.0
 seaborn>=0.9.0
 reno>=3.1.0;python_version>'3.5'
-Sphinx>=1.8.3
+Sphinx>=1.8.3,<3.1.0
 sphinx-rtd-theme>=0.4.0
 sphinx-tabs>=1.1.11
 sphinx-autodoc-typehints