update_requirements (#771)

docs/requirements.txt

-sphinx==2.1.2
-sphinx-argparse==0.2.5
-sphinx-markdown-tables==0.0.12
-sphinx-rtd-theme==0.4.3
+sphinx==3.1.2
+sphinx-markdown-tables==0.0.15
+sphinx-rtd-theme==0.5.0
 recommonmark==0.6.0
+sphinx-serve==1.0.1
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==1.0.3
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.4