Pin Sphinx doc generator version to "3.x" because recent version 4 breaks some extensions.

Author: plankter

poetry.lock

@@ -42,7 +42,7 @@ myst-parser = "*"
 pylint = "*"
 pytest = "*"
 pytest-cov = "*"
-sphinx = "*"
+sphinx = "3.*"
 sphinx-autodoc-typehints = "*"
 sphinx_paramlinks = "*"
 sphinx_rtd_theme = "*"