change: remove remaining instances of python-dateutil pin (#1235)

Author: laurenyu

tox.ini

@@ -97,8 +97,6 @@ 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
-# TODO: Remove pip install python-dateutil==2.8.0 once botocore no longer requires a pinned version:
-# https://github.com/boto/botocore/issues/1872
 deps =
     Pygments==2.2.0
     setuptools<40
@@ -113,9 +111,7 @@ deps =
 # 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
-# TODO: Remove pip install python-dateutil==2.8.0 once botocore no longer requires a pinned version
 commands =
-    pip install python-dateutil==2.8.0
     pip install --exists-action=w -r requirements.txt
     sphinx-build -T -W -b html -d _build/doctrees-readthedocs -D language=en . _build/html