nox > * lint_setup_py: failed in Kokoro

[skuruppu]

All Kokoro build seem to fail with this error. Example failure 1, 2.

Logs surrounding the failure:

nox > Session blacken was successful.
nox > Running session lint_setup_py
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/lint_setup_py
nox > python -m pip install docutils pygments
nox > python setup.py check --restructuredtext --strict
running check
warning: check: Duplicate implicit target name: "contributing".

error: Please correct your package.
nox > Command python setup.py check --restructuredtext --strict failed with exit code 1
nox > Session lint_setup_py failed.
nox > Running session docs
nox > Creating virtual environment (virtualenv) using python3.8 in .nox/docs
nox > python -m pip install -e .
nox > python -m pip install sphinx<3.0.0 alabaster recommonmark
nox > sphinx-build -W -T -N -b html -d docs/_build/doctrees/ docs/ docs/_build/html/

Affecting merging of #604.