Freeze sphinx-doc deps
Freeze sphinx-doc deps

In the 3.1.1 we have:
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Warning, treated as error:
autodoc: failed to import function 'simplify.hyperexpand.add_formulae' from module 'diofant'; the following exception was raised:
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.3/x64/lib/python3.8/site-packages/sphinx/util/inspect.py", line 324, in safe_getattr
    return getattr(obj, name, *defargs)
AttributeError: 'function' object has no attribute 'hyperexpand'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.8.3/x64/lib/python3.8/site-packages/sphinx/ext/autodoc/importer.py", line 71, in import_object
    obj = attrgetter(obj, attrname)
  File "/opt/hostedtoolcache/Python/3.8.3/x64/lib/python3.8/site-packages/sphinx/ext/autodoc/__init__.py", line 246, in get_attr
    return autodoc_attrgetter(self.env.app, obj, name, *defargs)
  File "/opt/hostedtoolcache/Python/3.8.3/x64/lib/python3.8/site-packages/sphinx/ext/autodoc/__init__.py", line 2078, in autodoc_attrgetter
    return safe_getattr(obj, name, *defargs)
  File "/opt/hostedtoolcache/Python/3.8.3/x64/lib/python3.8/site-packages/sphinx/util/inspect.py", line 340, in safe_getattr
    raise AttributeError(name)
AttributeError: hyperexpand
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diofant/diofant