Pin back docutils until theme is compatible

Upstream issue: https://github.com/readthedocs/sphinx_rtd_theme/issues/1115

Change-Id: I692fa425e9e195c1e74cac709cc39a59adc5839d
diff --git a/requirements.txt b/requirements.txt
index 16cdffa..fef9072 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,7 @@
 Sphinx~=3.1.1
 doc8~=0.8.0
 reuse~=0.11.1
+docutils==0.16
 sphinx-rtd-theme~=0.4.3
 sphinxcontrib-spelling~=4.3.0
 git+https://github.com/zdw/sphinx-multiversion@ec7c01cdaf2f9241245e84483dfb9bc0d0dffc62