Fix Sphinx documentation building

[woop]

What this PR does / why we need it:

Sphinx 4.0.0 breaks documentation building on master (also seen here datalad/datalad#5650). This PR prevents 4.0.0 from being used.

Does this PR introduce a user-facing change?:

NONE

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: woop

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[codecov-commenter]

Codecov Report

Merging #1563 (6a85998) into master (1436a02) will decrease coverage by 6.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1563      +/-   ##
==========================================
- Coverage   83.81%   77.49%   -6.33%     
==========================================
  Files          65       64       -1     
  Lines        5628     5496     -132     
==========================================
- Hits         4717     4259     -458     
- Misses        911     1237     +326     

Flag	Coverage Δ
integrationtests	?
unittests	77.49% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdk/python/feast/infra/offline_stores/bigquery.py	32.11% <0.00%> (-62.05%)	⬇️
sdk/python/tests/fixtures/data_source_fixtures.py	38.88% <0.00%> (-61.12%)	⬇️
sdk/python/tests/test_cli_gcp.py	40.74% <0.00%> (-59.26%)	⬇️
sdk/python/tests/test_feature_store.py	60.52% <0.00%> (-39.48%)	⬇️
sdk/python/tests/test_historical_retrieval.py	63.09% <0.00%> (-35.72%)	⬇️
...hon/tests/test_offline_online_store_consistency.py	74.72% <0.00%> (-25.28%)	⬇️
sdk/python/feast/registry.py	58.77% <0.00%> (-20.18%)	⬇️
sdk/python/feast/infra/offline_stores/helpers.py	75.00% <0.00%> (-16.67%)	⬇️
sdk/python/feast/data_source.py	67.94% <0.00%> (-10.90%)	⬇️
sdk/python/feast/feature_view.py	81.25% <0.00%> (-6.25%)	⬇️
... and 5 more

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