Add sphinx_rtd_theme to documentation dependencies in setup.py

Fixes: #5781

setup.py

@@ -118,7 +118,7 @@ for more information.
     extras_require = {
         'test': ['nose', 'coverage', 'requests', 'nose_warnings_filters',
                  'nbval', 'nose-exclude', 'selenium', 'pytest', 'pytest-cov'],
-        'docs': ['sphinx', 'nbsphinx', 'sphinxcontrib_github_alt'],
+        'docs': ['sphinx', 'nbsphinx', 'sphinxcontrib_github_alt', 'sphinx_rtd_theme'],
         'test:sys_platform != "win32"': ['requests-unixsocket'],
     },
     python_requires = '>=3.5',