Pin sphinxcontrib-blockdiag version

We need to use sphinxcontrib-blockdiag 1.5.5 for now
as it was the last working version compatible with the
rest of our tools

Issue-ID: DOC-576
Signed-off-by: Jessica Wagantall <jwagantall@linuxfoundation.org>
Change-Id: Iac4c9d588f0876408bd0a646b541c0fd5f8f6c55
diff --git a/etc/requirements.txt b/etc/requirements.txt
index 35ddb19..d75e59d 100644
--- a/etc/requirements.txt
+++ b/etc/requirements.txt
@@ -1,11 +1,11 @@
 tox
-Sphinx==2.3.0
+Sphinx
 doc8
 docutils
 setuptools
 six
 sphinx_rtd_theme>=0.4.3
-sphinxcontrib-blockdiag
+sphinxcontrib-blockdiag==1.5.5
 sphinxcontrib-needs>=0.2.3
 sphinxcontrib-nwdiag
 sphinxcontrib-seqdiag