Exclude docutils!=0.21 as a dependency (#615)

This release is bodged, and causes Poetry to fail when it tries
to fetch the tar:

python-poetry/poetry#9293 (comment)

It is being tracked:
pypi/warehouse#15749

A fix is inbound here:
pypi/warehouse#15795

This is a hotfix for this particular package, until the fixes from `pypi/warehouse` are out. The `post` notation is very unique: pypi/warehouse#15749 (comment) and is being used in 0.004% of the releases.

pyproject.toml

@@ -85,6 +85,7 @@ typing-extensions = "4.9.0"
 pytest-mock = "3.12.0"
 pyspark = "3.5.0"
 cython = "3.0.8"
+docutils = "!=0.21"
 
 [[tool.mypy.overrides]]
 module = "pytest_mock.*"