infra: upgrade Sphinx to 3.1.1

This commit also consolidates the doc dependencies to doc/requirements.txt,
removes some unnecessary dependencies, and upgrades sphinx-rtd-theme to 0.5.0.

Author: laurenyu

doc/requirements.txt

@@ -1,4 +1,2 @@
-sphinx==2.2.2
-numpy
-scipy
-requests==2.20
+sphinx==3.1.1
+sphinx-rtd-theme==0.5.0

tox.ini

@@ -98,20 +98,6 @@ commands =
 [testenv:sphinx]
 basepython = python3
 changedir = doc
-# Based on: https://github.com/rtfd/readthedocs.org/blob/8f0c78dde5edcc85acf90462a8518735a25482d3/readthedocs/doc_builder/python_environments.py#L263
-install_command = python -m pip install --upgrade -I {packages}
-# Based on: https://github.com/rtfd/readthedocs.org/blob/8f0c78dde5edcc85acf90462a8518735a25482d3/readthedocs/doc_builder/python_environments.py#L280
-deps =
-    Pygments==2.2.0
-    setuptools<40
-    docutils==0.13.1
-    mock==1.0.1
-    alabaster>=0.7,<0.8,!=0.7.5
-    commonmark==0.5.4
-    recommonmark==0.4.0
-    sphinx<1.8
-    sphinx-rtd-theme<0.5
-    readthedocs-sphinx-ext<0.6
 # pip install requirements.txt is separate as RTD does it in separate steps
 # having the requirements.txt installed in deps above results in Double Requirement exception
 # https://github.com/pypa/pip/issues/988