pin sphinx version becuase of sphinx bug in 3.2.0

[jandremarais]

Description

Because of this bug, sphinx-doc/sphinx#8074, we can't use sphinx 3.2.0 to build docs for latest PyDP with docstrings. It results in

Extension error:
Handler <function _process_docstring at 0x7f5a23643310> for event 'autodoc-process-docstring' threw an exception (exception: module, class, method, function, traceback, frame, or code object was expected, got instancemethod)
Makefile:56: recipe for target 'html' failed
make: *** [html] Error 2

To reproduce - inside PyDP (dev branch) and in a clean virtual environment, run:

pip install .
pip install sphinx-rtd-theme
cd docs
make html

Thus without restricting the version of Sphinx, in docs/requirements.txt we run the risk of install 3.2.0 and therefore building the docs will fail. Thus the fix in this PR restricts the version of Sphinx in docs/requirements.txt to be no greater than 3.1.2.

Affected Dependencies

Sphinx

How has this been tested?

In a new python environment with only the python-dp in the current dev branch installed, running:

cd docs
pip install -r requirements.txt
make html

succesfully builds the docs.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the OpenMined Styleguide
• I have labeled this PR with the relevant Type labels
• My changes are covered by tests

[chinmayshah99]

Nice catch!