doc: Limit sphinxcontrib versions to <2.0.0 as we use Sphinx 1.8.5

docs/requirements.txt

@@ -5,9 +5,9 @@ sphinx>=1.8.4
 breathe==4.11.1
 sphinx-rtd-theme
 sphinx-notfound-page
-sphinxcontrib-blockdiag>=1.5.5
-sphinxcontrib-seqdiag>=0.8.5
-sphinxcontrib-actdiag>=0.8.5
-sphinxcontrib-nwdiag>=0.9.5
+sphinxcontrib-blockdiag>=1.5.5, <2.0.0
+sphinxcontrib-seqdiag>=0.8.5, <2.0.0
+sphinxcontrib-actdiag>=0.8.5, <2.0.0
+sphinxcontrib-nwdiag>=0.9.5, <2.0.0
 recommonmark
 future>=0.16.0 # for ../tools/gen_esp_err_to_name.py